Glop can only be engaged (and, therefore, looted/pick pocketed) when players are on the aforementioned quests.
Glop is immune to snares and stuns.
Technically every time players encounter Glop on the daily quest he is actually the son of the Glop players previously killed.
Glop can't be engaged until after Earthmender Norsala walks down and talks to him. If you have a DoT or some kind of AoE over time, you could try to time it so Glop will take some damage before running off.
Rogues with daggers can use sprint to keep up a bit and [Backstab] Glop while he is running.