Actually, there are many, many answers to that question. Firstly, they may have simply had other GFs that you couldn't draw. As Garden students, this is likely.

Secondly, as members (sort of) of the Galbadian military, they could have adopted the Galbadian style of para-magic--that is, without GFs. Personally, I have theories about how they did it, but it is confirmed in canon that they did use para-magic without GFs in the Galbadian Army, so that's enough to state that it's possible, with the proper training and/or equipment.

The reason Garden doesn't use that method is that GFs are more powerful.