Refund for medical services never performed

1. You agreed to the terms that when you cancelled you would be refunded as a gift card.
2. You cancelled. You got a refund as a gift card. Exactly as stated in the terms.
3. You are surprised by this.

Yes of course it's legal. It's legal for them to refund you nothing if those were the terms you agreed to. Plenty of businesses work on the basis of no refunds.

Is there anything you can do to get your money back? No, but you can rebook the appointment and get the test you paid for.

A chargeback will not work. The company has not "failed to provide the service". They were willing and able to provide the service but you declined it by cancelling the appointment. And presumably they are still willing to provide it at a time of your mutual convenience. If they are not able to reschedule that is a different matter.

If there is a genuine medical reason you don't need the test any more ask nicely if they will refund in full.