GM transmissions do not require RTV, Anywhere!
Go to a local transmission shop & get the correct O-ring.
If you want the Detent to work & not drop the Pan again to change the Detent Solenoid, Here is a whole kit, Passthough, O-ring, Detent Solenoid, & Solenoid Gasket.
http://www.ebay.com/itm/TH400-3L80-D...-/310580992730