4T60e dropping pan will get you about 7 quarts, give or take.
I recommend Amsoil or Mobil-1 ATF. Nothing comes close and will last much much longer and tranny will be much happier.
If you're using a different ATF than the ones I mentioned, it be prudent to flush it twice. With at least 2 weeks between flushes.
The second flush instead of dropping pan again, you can shove flexible hose of appropriate size down the dipstick and take the fluid out with a transfer pump. I've done this many times saving me tons of work. Since you already changed the trans filter on the first drop, you won't have to change it right away anyways.
Re-use the original gasket and don't think about using anything else except the original (re-usable) trans-pan gasket, or a new gasket from the dealer.
Aftermarket gaskets that come with the filter kit are a joke and never work !