The 23-year-old played as a trialist in Saturday's friendly with AEL Limassol and will now boost the Firhill side's options ahead of their return to the top-flight.
Gabriel also featured in a closed doors game with Cowdenbeath and did enough to impress Thistle boss Alan Archibald.
The player told the club's website: "I'm very happy with the opportunity. Hopefully it will be good for everybody - for the team and for me.
"I came over last week and the club just wanted to make sure that I was in good shape and I had a good way of playing. I'm very happy to sign here."