It's difficult to know where to begin when optimising your Division 2 build. The game tells you very little, and discussing it with people online makes it sound far more complex than it really is. What you need to be aware of, before you begin to optimise, are several key things:

• Creating a "perfect" build is still in the hands of the RNG Gods. Expect to farm a lot if you want the best of the best. 
• Don't expect to obtain an optimised build over night. It takes many hours of farming content to get the items you'll need. 
• Take time to understand Attributes and Talents, and how they function together. 
• Have a specific build in mind, by answering the following question:
  • Which weapons do I want to use?
  • Which specialization is right for me?
  • What sort of playstyle do I like? 
• Ignore Gear Score. Your focus is on synergizing your build in WT5. Your Gear Score level is largely meaningless if your build is just a random collection of high item levels. 

Gear Score

Gear Score is the equivalent of your item level, and is an average taken across all your core items (excluding mods). Each item, such as a weapon or piece of armor, has a Gear Score. This Gear Score increases up to a base of 500 in World Tier 5. A fully optimised build, and when recalibrating items, will see you surpass an average of 500 (items gain Gear Score when recalibrated to a higher Attribute). However, Gear Score - as noted above - is largely meaningless if your gear isn't optimised.

Brand Sets

Every item in The Division 2 - excluding mods and weapons - has a Brand Set. These are seperate to Gear Sets (which are six items) and instead, rely on only three being obtained. Each equipped item from a Brand Set will provide a bonus, based on the number equipped. Equip all three, you'll obtain one of the bonuses. 

If you equipped two pieces of the "Providence Defense" Brand Set, you'd gain +10% Skill Power and 8% Health.

Brand Sets tend to have a specific focus, such as "Douglas and Harding" focusing on offensive bonuses (it provides accuracy, critical hit damage, and ciritcal hit chance), while "Gila Guard" is defensive for armor and environmental protections. It's important to remember that the bonuses Brand Sets provide DO NOT contribute to your Attributes. They provide a bonus that's entirely independant, but which still contributes to your total Stats. 

Lastly, Brand Sets - despite their general focus - can still roll random attributes and talents. 

Attributes

There are three Attributes in The Division 2, and each is represented by a symbol. 

• Red Circle (Offensive): Offensive Attributes which covers weapon damage, critical hit chance, critical damage, specific weapon damage (such as Shotgun or Rifle), damage to elite enemies and headshot damage. 

• Blue Shield (Defensive): Health and armor, as well as environmental protections. 

• Yellow Battery (Utility): Skill power and cooldown reduction. 

Every item will have at least one attribute, up to a maximum of three. In addition, each Attribute and its roll on gear is random, meaning that the values you receive from the same gear and category can vary. For example:

The item comparison above (two Providence Defense masks) shows that their brand sets remain the same:

• 10% Skill Power
• 8% health
• 5% weapon damage

However, their Attributes are very different. One has two defensive Attributes, while the second (right) has just one, alongside an offensive. Despite differing item levels (499 - left, versus 495 - right), the one on the right is unquestionably stronger. Its Talent is better (we'll discuss this below), armor higher, while the offensive Attribute which boosts damage against Elite enemies by 29% is also significant. 

While it might appear logical to just pursue gear with as many Red attributes as possible (gotta get the damage!), it's not quite that simple, and that's largely because of Talents.

Talents

There's a varierty of Talents, and these effect both weapons and items. Each can have a total of three, and each of the three have different Talent pools. A level 1 Talent is the most powerful, the second tends to provide flat buffs for the weapona and/or item, while the third is circumstantial and provides a brief benefit based on whether your weapon is holstered, equipped, or if you perform an action (such as rolling).

Items and weapons have different Talent pools, while this is further divided for weapons into category (Sub Machine Guns gain access to different Talents in comparison to Rifles). There are many Talents (full list here) for items and weapons, and the roll on each is random. Some will have only one, others two three. 

In addition to this, certain Talents have specific Attribute requirments in order to be activated, and these tend to be a Level 1, and the most powerful. For example: 

The backpack highlighted above (Horo CarryBack) has two Talents. The Level 1, "On the Ropes", requires 7 or more Utility Attributes. Any less, and the Talent won't work (more is fine). In comparison, the Hard Hitting Level 2 Talent, "Hard Hitting", is passive, and will work irrespective of your Attributes. 

As you can see from the far left side of the image above, in total my character has:

• 3 Offensive Attributes
• 7 Defensive Attributes
• 7 Utility Attributes

These Attributes are gained across all items, as well as Mods (but remember, Mods don't affect Gear Score). 

The complexity in optimising your build comes from:

• Finding an item with high base Attributes.
• Finding an item with the right Attributes for the Talents you're aiming for. 
• Finding an item with a high amount of Talent slots. 
• Finding an item with the right configuration of all the above, so that you only need to Recalibrate one wrong Talent and/or Attribute.
• Synergising all your Attributes, across all items, to ensure all Level 1 Talents work.

Unsurprisingly, there's a lot of RNG here, and a great deal of inventory management when it comes to looking at items and comparing them to what you have. Fortunately, Recalibration does help. 

Recalibration

Recalibration is a simple system of taking something good from one item you don't want, and plonking it onto an item you do. The only catch is the fact that you can only Recalibrate one Attribute or Talent on a single item, though you can then swap the Recalibration out, but only in the same slot (Talent/Attriubte). 

Recalibration allows you to effectively find a weapon or piece of armor that's not quite perfect, and make it perfect. The only catch is the fact the weapon you're Recalibrating has to be pretty bloody perfect to start with. That said, Recalibrating can increase an items level, and also ensures that you can bring brilliant Talents from a rubbish weapon, to make what you have even better. 

While Recalibrating is important for weapons, I'd say it's far more important for items as you've the opportunity on those to choose Attributes and Talents, but be conscious of the fact you cannot change Attribute type (you can't swap an Offensive Attribute for a Defensive one). 

If you can, try to favorite items and weapons with good Talents/Attributes, that only require one single change to make them amazing. Also remember to factor in active Talents and their Attribute needs, and to research which Talents are best.