Well, quite early really. As scimitar was just a +1 when gaius found it, it was his first magic weapon really. A weapon which helped start build his legend and did his first steps in his ultimate goal end the demon invasion and first villains slain etc. But then he got fortune enough get a blade crafted which he felt most comfortable with but kept old one as memento (and as back up weapon if his main weapon would be stolen or broken). But if Crassus paid attention to any gift mails what Gaius send to his offspring he would remember Gaius sending that blade to Gilbert. Altough blade is just a +1 weapon it had some dinstinct decoration and marks to it.