So these are common problems I see in most maps that I figured I would try to make known instead of leaving feedback every time I see it.
If you are using the ABS Resource Pack to make setup gates, note that the door brush entities are not extended to the frame. Also the gates are not solid.
What this means is that if you are sizing your doorways to accommodate the frame, then you should be extending the door brush entities as well. Otherwise, make your doorways as big as the door brush entities (less work and is how it is intended). Not doing these things will allow players to shoot through the gaps between the doorway and the actual door brushes.
For the small setup gate the doorway should be 128 x160.
For the wide setup gate the doorway should be 192 x 160.
For the tall setup gate the doorway should be 156 x 204. Gross.
Again this is only if you are using the ABS Resource Pack. If you are manually making setup gates or using different doors, this won't be a problem unless you make it one.
If you use metal grate textures, there are a couple of textures that allow explosive damage to go through. What these textures have in common are a property in their respective .vmt files, %compilepassbullets. If you aren't sure, check!
These three are ones that do for sure: metalgrate011a, metalgrate011b, and metalgrate013a.
There are alternative textures for 011a and 013a that do not allow damage to pass through. These are reck_metalgrate011a and reck_metalgrate013a2 respectively. Remember this before you make that capture zone made out of grates.
If you are using the ABS Resource Pack to make setup gates, note that the door brush entities are not extended to the frame. Also the gates are not solid.

What this means is that if you are sizing your doorways to accommodate the frame, then you should be extending the door brush entities as well. Otherwise, make your doorways as big as the door brush entities (less work and is how it is intended). Not doing these things will allow players to shoot through the gaps between the doorway and the actual door brushes.
For the small setup gate the doorway should be 128 x160.
For the wide setup gate the doorway should be 192 x 160.
For the tall setup gate the doorway should be 156 x 204. Gross.
Again this is only if you are using the ABS Resource Pack. If you are manually making setup gates or using different doors, this won't be a problem unless you make it one.
If you use metal grate textures, there are a couple of textures that allow explosive damage to go through. What these textures have in common are a property in their respective .vmt files, %compilepassbullets. If you aren't sure, check!

These three are ones that do for sure: metalgrate011a, metalgrate011b, and metalgrate013a.
There are alternative textures for 011a and 013a that do not allow damage to pass through. These are reck_metalgrate011a and reck_metalgrate013a2 respectively. Remember this before you make that capture zone made out of grates.