with the power gem and a GL ring he could and would pawn Galactus. there is a "lot of things he could do with the power gem alone to GL..
aside from that if we entertain the idea of the DC/MU crossover a power ring added to Thanos powerset turns him into an abstract lvl type being who can tear down reality negating existence by simply slashing at it with his fingers.
the power gem is an infinite power source it has run machine's in the bast from solar busting canons to the infinite thrall created through machinery by Man-Beast.
Thanos could simply feed the ring a steady flow of constant power by will alone.
or he can command all power from galactus to be drawn into the gem
Let's not forget that the PG would make Thanos almost immune to punishment. Especially if he's had it for 2 weeks and been tapping into it and acclimating him to it. If Thanos's shields can make galactus go from well nurished to needing to feed... then Thanos would be fighting a weaker Galactus and this was just to get through Thanos's shields.
the gem wont run out of juice...
even dumb Drax was able to tank energy blast from Count Abyss a nigh infinite being, god of it's own universe..
Abyss stated even he couldnt match the might of the infinity power gem and instead reversed the power feed from Drax...
and this was a supreme being of his own universe.