Is it permissible to use the free gift balance I obtained due to a loophole in the website?

If the company giving the gifts allows the customer to receive them only once, then it is not permissible to use deceit to obtain the gift again. This is because it involves consuming wealth unjustly and violating the condition set by the granting entity, for the Messenger of Allah, peace and blessings be upon him, said: "Muslims are bound by their conditions."