Gabriel is a craftsman from Verden who has the distinction of being the inventor of miniature crossbows, possibly wrist-mounted. Gabriels were named after the inventor and marketed as ideal for personal defense from bandits. They became very popular. Naturally also with the bandits. One can only assume that Gabriel died quite wealthy (or given Sapkowski's sense of humour, a victim of his own invention!).
In the game, the Professor uses a miniature wrist-mounted crossbow, which is more than likely one of these weapons.