Don't say chosen baby's name out loud as if it was an object and then seemingly approve or disapprove.
Don't give unsolicited advice.
Don't grab baby and sit around in GC's house while DIL wants to nap but is still at the newest mother stage of not wanting to leave her baby out of sight.
Don't march over to neighbours to show off GC either with DIL trailing behind you or just left literally without her baby.
Don't exclaim how different things were forty years ago.
Don't offer to get mildewed pram from attic.
Don't spray overpowering perfume when near GC.
Don't ask 'but why is soother cream not recommended anymore, but why, but why, but why.......
Don't say 'we all thought you were having a boy' with sad face if a granddaughter arrives...........
I will think of more, lots more.