I played the game downloaded from GoG without any modifications.

Near the end I was surprised that my characters could level up higher than level 10, so unless the official patch had a level cap remover the GoG version must include an external mod.

Reaching level 12 just before the end I found out there is even rudimentary support for higher level spells , wizards get three level 6 spells, cleric gets heal and harm and the druid mass cure light wounds. Spontaneous cleric level 6 healing yields inflict minor wounds and and the druid level 6 spontaneous summoning via shift key shows an error message in the radial menu. XP scaling stops however, at level 11 my party didn't get less XP than at level 10.
There might be more modifications without me realizing, didn't play the game before. It surprised me that "Forge ring" is among the feats although it requires caster level 12.

So the question is, which modifications exactly are included in the GoG version ?
Are there any readmes which show what was changed compared to the original ?

Thanks for your answer. Do other platforms than GoG sell a different version of the game ?

As far as I know only GoG sells it these days.

For the sake of completeness

Is that bug that formats your computer fixed in the gog version?

I thought that was the later Pool of Radiance (no, not the one available on this site; the one that used 3rd edition rules, but apparently wasn't that good) that had the bug,

Well that explains the lack of bugs.

I thought all the reports on "all the bugs" in TOEE were exaggerating.