I have always found Auto trans gaskets to be tricky. The reason being they have some elements going against them. They seal a lot of light fluid and are in a vulnerable spot on a car. They use small bolts which can only be torqued a small amount usually like 30 in/lbs max. I have also found transmission pan bolt holes to sort of buldge out over time. Here are the steps I use to ensure a good seal.
Once pan is removed clean, check for flatness, straighten if necessary
Check bolt holes on the trans. I take a counter sink bit and use my cordless drill to make sure there is no buldge
Clean bolt holes on trans
Clean transmission bolts
Use a quality pan gasket- I like cork, but the rubber ones work well too
Install gasket, I dont really like silicone but if the pan is sketchy I will use a light amount
Use a drop of blue thread locker on all the trans bolts. This keeps them locked in under the light torque
Torque all bolts
This takes time but it works. The key is the thread locker. If the bolts get torques too much they blow out the gasket and that causes a leak.
Just my $.02