I have 6 through 9 on CD's (the "Platinum" set) and have played them through several times each. But when they all went on sale a few weeks ago, I got the GOG versions. (Just because it is a PIA to install them in newer Windows versions when I get a new computer.)
Now I am playing MM6. At first I only noticed one minor difference… I used to find the occasional (low-level) ring in a cook-pot. But I never find them in this one. But now… None of the “Dungeons” respawn. The outside “monsters” around New Sorpigal and Bootleg Bay do, but that is all.
I know that they all had different respawn rates.
For instance outside New Sorpigal takes 6 months after killing the last monster, and the inside ones used to take a year, but it has been over 2 years now and nothing is back. I can live without having to re kill low level opponents, but I miss have the barrels and treasure respawn.
Are these differences on purpose, or is there just a glitch in the GOG version? And do the other GOG versions (7-9) have the same lack of respawning?

It is just your luck. It isn't a common occurence to get rings from cookpot, they are quite rare and IIRC can't be acquired from cook pots by save-scumming either. Goblinwatch respawns in 2 years, timer starts from the day you first enter it (1st time or after respawn). It works like that in GoG' version too. Easy way to check - enter Goblinwatch, open door to the right and kill rat. go nuke some peasants and spend 2 years in prison. When you return back to Goblinwatch, rat will be back and alive. On my current slow playthrough (GOG version) I have Sorpigal respawned for 3rd time (according to its respawn timer). GOG versions are MM as is, without any changes, except music was converted into mp3-format, because current format was not playing music reliably. GOG versions have all official patches (no unofficial patches, which change gameplay, applied - to keep games in their original form). MM 6 has also unofficial patch from GrayFace, but it is because of couple game-breaking bugs in original. MM 7, 8 and 9 have unofficial patches for compatibility with Win7.

Edit: Also you need to change maps to push respawn if it is respawn time for your location.

Sorry to hijack this thread, but how do you get arrested and spend 2 years in prison?? I killed a few peasants sometimes (then reloaded), but never cam across any prison. Is it in every town?

When you kill too many peasants, or will make your reputation down to "Notorious" by some other means, when you enter any Castle of any Lord Council you will be sent to prison for 1 year. I just checked it myself (on GoG version). It respawns. According to intended respawn rules for this area. I guess you are mistaking this place for Castle Harmondale in MM 7 (which had a lot of goblins initially too).

Thanks, you were right. Two years to respawn LOTS of places. I think it must have just seemed like less because I spent longer traveling around. A few days by coach, boat, or shank’s mare = no actual playing time but lots of game time. This time through I tried something new 3 Archers (1 each for fire, air, and water) and 1 cleric. That let me have one specialize in (water) travel spells so I get around faster earlier in the game and can easily re-visit old areas in a shorter (game-)time frame.
And I’ll take your word about the stew-pot rings, but I swear that I almost always got one In New Sorpigal and another from the Cannibal pots in Bootleg. (Which for some reason always makes me think of Barbara Streisand... “People… People who eat People… are the yuckiest people... in... the world.”)
Sorry, that’s just the way my twisted mind works.

Prety sure you're supposed to get rings always in New Sorpigal and Bootleg Bay. There are some cookpots other places (Silver Cove among others if memory serves me right) which don't always yield, but in NS and BB, they definitely do. At the very least, more often tha not.

No I'm definitely talking about Goblinwatch from MM6. I used to store all my valued stuff down in the 'basement' area with the numerous chests situated inside of the walls there. It could be a pre-patch/pre-grayface bug though.