The PCs are currently adventuring in what they're calling "the Starship Warden," a bounded forest/swamp/jungle/killer vegetation area.
At the end of their botanical expedition ("Killing plants!"), they'll presumably return to Felltower . . . a couple months after they entered the gate.
We had this situation come up with Olympia, which even had a delve in between.
This one won't have the delve, but it'll be months for sure between delves.
What does that mean for restocking?
It means it will happen.
When the PCs return to Felltower, either by the same gate or some different way in - or if they somehow get back to town without going through Felltower, the next time they delve within.
The orcs will have responded to a couple months of quiet. Monsters restrained by portal they propped open might move into new areas. Randomly settling creatures may nest between their location and the exit.
Even if the gate took them to a place on "Earth," clearly time was lost in the transit from one place to another. It felt instant to them, and it was, subjectively, but objectively time passed. Gates are weird that way, clearly.
It makes gate travel rough, but gives it a lot of versimilitude and makes for a lot of fun. I recommend this way of doing things.