The Gargantuan in the SM camp by Rivet City... got stuck a lot. Especially if he ran up the hill towards the back area with the sleeping mats, med kits and ammo spawns. I could not get him to come out of the camp to come play with me, so I had to slowly kill him where he got stuck. All the regular sized SMs could get to me outside the west of the camp where the benches and parking meters are.
The Gargantuan (and then a Behemoth spawn) at Jefferson Memorial constantly gets hung up on the two steel structures that pass over the top of the raised walkway. They'll run up against it for a long while, then magically warp through one, then get stuck on the 2nd one for a while, then warp through that one. Behemoths are also too tall to pass under that steel structure (about 10% too tall).
I need to start carrying more Nuka-Grenades and Bottlecap Mines...