I just noticed I am unable to install this game in my D:\Program Files (x86)\ folder where all my other GOG games are living.

I know the new installer defaults to a different location, and I also know there were some issues with the . in GOG.com for some older games, but this is a new game, and the offenders are the parenthesis in (x86)...

Is there really an issue with this game which does not allow this, or is it an oversight in the installer? If the latter I would really like this to be updated, as I like to keep all my programs in the Program Files folders, which until now never was a problem.

*edit: the Steam version just happily installed to D:\Program Files (x86)\Steam\etc... so at least it's not the game per se.

Some older games can't handle non-alphanumeric installation paths, so for a period of time ALL the new installers disallowed those characters even if the game is fine with them. Giana Sisters is one of those installers, unfortunately. It's not the game, but rather an over-zealous restriction of the installer. Fortunately only a few installers suffer from this problem unnecessarily, and eventually even those will be fixed.

On the other hand, some games tend to work better when installed outside of Program Files, so I normally don't install them there anyway.

Ah, thanks for confirming / clearing that up.

By the way, as long as it's not the Program Files folder on your main system drive there are no negative effects. At least not for me so far...