A few things:
1. in GLAX, it's drilled into players from the first day that participate in the sport that it's illegal to hit an opponent in the head. In youth GLAX, it's a foul if you even attempt to check a stick above the ball holder's shoulders. In high school, checking above the shoulders is allowed, but it's still a mandatory card if your stick hits an opponent in the head
2. It's not as though hitting an opponent in the head never happens -- it's probably the most common yellow card in high school GLAX. But 99% of such cards are issued when a defender attempts to check an attacker's stick, and inadvertently hits the attacker's head (or neck). As such, it's almost always a glancing hit or a soft hit. I'm around the game quite a bit, and I'm pretty sure I've never seen a hit to the head that caused a cut to the fouled player
3. If an attack player hits an opponent during a follow through on a shot, it's a mandatory yellow card for dangerous propel in high school GLAX
4. Between the above #3, as well as the shooting space rule in GLAX that I've previously mentioned in this thread, the situation you encountered (getting hit in the chin with full force on a follow through, causing a serious gash as well as a lasting scar) is not impossible in GLAX, but it's extremely rare