He might be, therefore he is? That's flawed logic. He could be the start of the clan. I don't think he would be, though. His beloved was already dead, and Gabe didn't seem to be the one to snog some other broad. They could've had one beforehand, but I don't know.
Then again, there might not even be a Belmont clan in this continuity.
The same could be said of Leon. He loved Sarah dearly, and due to mathias' machinations, she had to die by his own hands, and yet he had himself some family regardless.
Gabriel grieves, and later starts over. Its quite possible. Also, all because he is a Belmont in "name only" honestly has very little significance. he is this continuities Belmont. and if he has family and kids down the line, he would be the founder of the Belmont clan.
you seem to be basing the idea "he's only a Belmont by name" due to the fact that he is of Cronqvist blood, and that compared to the Belmonts of the other continuity, he would NOT be a Belmont. However, the other Belmonts dont exist here, and as far as names go, the Cronqvists are likely insignificant here, probably only for the link to LoI having a Cronqist that eventually becomes Dracula. He himself named himself that due to his love of mountains. He is the first Belmont of the LoS universe.