I just recently discovered I can play old games like AvP online again which is amazing, but as I am new to this platform I am trying to find a list of older games that are supported like this. Is there such a list?

There are no such official listings on GOG, but the community has plenty. Here's a list courtesy of Magog, GOG's unofficial search engine made by user mrkgnao:
[url=http://www.an-ovel.com/cgi-bin/magog.cgi?ver=717&scp=gdspuio&dsp=ipgfsorlcmbahTP&ord=&flt=man~m~&opt=nr]http://www.an-ovel.com/cgi-bin/magog.cgi?ver=717&scp=gdspuio&dsp=ipgfsorlcmbahTP&ord=&flt=man~m~&opt=nr[/url]

Edit: Better edit in a ping so he actually sees this thing.