If some games had bug issues or other technical problems from its original release, does GoG make any changes to them? Or do the games always come as is, with no updated changes? Just wondering.
GoG patches to the latest version and most of the time (but not always - see Settlers V) also includes the addons or bundles games (like Descent 1+2, Tex Murphy 1+2, Might and Magic Sixpack etc).
There is no extra fixing of bugs. If there are some left even with the latest patches, they will stay. For example, see the thread about Sacred Gold I made a short while ago. http://www.gog.com/en/forum/sacred_gold/original_bugs_fixed/10
However, when possible, you can expect someone to link to inofficial patches, sometimes even a GoG staff worker, like on Giants: Citizen Kabuto. http://www.gog.com/en/forum/giants_citizen_kabuto/mods/_/1
Sadly I had a bluescreen with the mods because of a graphics card error - the only one I had with that card -, and some weeks later my PC (after 18 months of working improperly - or 52 months if you include all the time since I bought it - never again I will buy a pre-manufactured PC!) finally died.
Now I got another PC but had to use the old graphics card. However, with my new one I have now - got it a few weeks ago for christmas - I might give it another try. The game seems to be great and the mods linked by the GOG staff worker really enhance the graphics greatly.