Be the best Hero that you can be on Watchpoint: Gibraltar.

Any Hero can have success on this map, but the following are overviews of choices that might make sense for you.  

• D.Va can be useful while Defending as her mobility allows her to harass healers and squishies slowing the push. 
• Genji - There is a lot of vertical space for Genji to use.  In the right hands he can be played in either Attack or Defense, but if you aren't a great Genji player then he should be avoided. 
• Junkrat can also make use of the vertical space to rain down his bombs.   He's a solid choice on both Attack and Defense. 
• Lucio is a strong pick, A++ on both Attack and Defense.
• Mei is a strong choice on Defense if you team has enough DPS.  If not, avoid her.   Freezing people who nobody can kill doesn't help your team. 
• Mercy is an A++ choice on both Attack and Defense.   
• Pharah is great on this map as Attack.   On Defense she can contribute up until the final point where there are better choices.  
• Reaper is a solid choice on both Attack and Defense due to the great flanking and vertical space of this map. 
• Reinhardt is almost necessary on Attack against some compositions.   He's also an excellent choice on Defense as he can slow down the push. 
• Roadhog is above average on Defense, but challenging to make work on Attack without a coordinated team. 
• Soldier: 76 is the baseline.  You can play him on any map.  Attack of Defense. 
• Symmetra can be useful on Defense at Point A, but should be avoided otherwise. 
• Torbjorn can have his moments on Defense, especially at Point A and against weak teams as you move through the middle of the map. 
• Tracer is excellent at harassing Defenders throughout this map.
• Winston is an excellent choice if you are faced with multiple squishies or snipers.   He can get to them and make them pay. 
• Zarya is a solid choice on Attack or Defense at all points throughout the map. 

Point A is relatively easy to hold, but once broken the second point is almost always taken immediately.    The next defensible spot that causes issue is just inside the doors from the second checkpoint.   The walkways and S turn force the Attackers into a very vulnerable spot for a long period of time.   Many games are won here. 

Once this checkpoint is lost it is almost inevitable that you will be making a last stand at the destination.    Your spawn point is almost on top of it though, so any 1 for 1 trade is usually a good choice. 

Cynical Nerds have an excellent video guide to this map.  

Break the Game's  Tips for Attacking on Watchpoint: Gibraltar

Using the One-Way Window on WP:Gibraltar