when an update comes out for a game that i have already bought, will i be able to install it? or is am i stuck with the version i bought when i bought it?

The games here are so old the chances of an update are slim to none. If they do however it will probally be you downloading the installer again.

Also, chances are GoG will do it's best if it's an official update, to include it in their package, I'm sure.

True, the only game that may see an update is Original War since its still supported by the original developer. I believe they will update their installers if there is a new patch. I mean they have updated to the Redneck Rampage installer to add content already.

The only update I can see you paying for would be any 'expansion pack' update.
It's quite possible that any official patch can be applied over the top of the existing GoG version, but if not, I'm quite sure it will be available as a seperate GoG download.