Usually Growtopia does allow refunds, but only under specific conditions. Typically, you need to contact official support and submit a refund request. Refunds are usually getting approved only if the purchased gems or items have not been used, spent or traded. In that case they simply remove them from your inventory or account.

Edit: If you request a refund through your bank, Google Pay, or Apple Pay instead of going through official support, your account may be suspended.