The bolt gaskets are thin black washers.
Some GM trannies have pump bolts with a groove just under the bolt head that accepts an o-ring.
I've taken some apart that have had both installed. Only the proper one should be used on the proper bolt.
Take a bolt out and check it.

Its almost impossible to pinch or knick the large perimeter rubber pump seal when installing. The gasket behind the pump will not cause external leaks.

Check the seal at the dipstick. Oil leaks can seep a long way and appear they are coming from another spot.