Problem is, you are not OP's boss and you have absolutely no idea what info regarding to this situation is in the OP's contract or other formal training that has a paper trail.
I'd be amazed if the OP is the first member of staff to do this, the company should need to explain why they have not closed the loophole if they do not approve of staff buying gift cards and then buying products with the cards, both with discounts, if there is nothing with a paper trail disallowing it.
And as pointed out by others, has the OP been given taken down the route of disciplinary without following employment law procedure?
Formal letter ahead of meeting stating company's reason for wanting a formal meeting?
OP given opportunity to prepare for meeting?
OP been given opportunity to take someone with them into meeting?
etc.