Sorry to come to this conversation late. In my personal opinion, I would say that from a GURPS support standpoint--all unofficial, technically--the top three in order are: (1) FoundryVTT with GURPS Game Aid (by Chris Normand and co.); (2) Fantasy Grounds Unity; and (3) Roll20.
I've used all three and run a DFRPG game using FoundryVTT--it is great and the Game Aid is excellent. Really, really excellent. It was the last one I used and currently use, and that's it--I'm not going back to others because I like this one so much.
Ronnke (not sure of his real name) has put together a very good "system" for Fantasy Grounds, and it is very robust. It's an excellent system, in fact.
Roll20 is solid but has the least amount of support (i.e., I can write a template macro or script to do things...kind of...that are done in FoundryVTT or FGU). That's not to slight the guys (MadCoder, I believe) who have worked on the GURPS Character Sheet and such. It works, and I use it for a game where I'm a player, not a GM.
I recommend checking out the GURPS Discord server (
https://discord.gg/SY5Xuaah), and there is a dedicated channel for FoundryVTT (as well as Roll20 and Fantasy Grounds).