>>9032758
Like most anons said, probably just periodic training, Black Mesa was huge, government company, also doing high-tech weapons and defense systems. For example, even if you're a civilian in military IT, you still have to get basic training in many countries.
Not to mention, what happened in HL1 was all planned, the G-Man knew what Gordon will have to do before the Resonance Cascade, they could have easily added weapons training for that purpose even if it was not standard and claim it is. Nobody would have known better.