I've been waiting on GoG support to help me cancel an old account that I ended up linking with my steam's and I'm unsure how to check which games are avaible through the service, would appreciate some explanation on how it works and if Underrail is avaible now for it as well.

GOG Connect is time-limited promotion which occurs during major sales or specific occasions. Your can read more about it here. There are no games right now on GOG Connect.