The Gucci sunglasses are so fashionable at the design. If it is a little bigger than your actual size of head, you'd better use the screwdriver to adjust the nose pad position. Then you will find that it is suitable for you. The way is so easy that using the screwdriver to make the nose pad inward a little. That will be OK.
You can try to tighten it by yourself. You should find a tiny screw driver, to tighten the screws on both sides by turning them until you feel resistance. Then , you can try to wear it to feel the degree of resistance.
If your sunglasses remain loose after tightening the screws, you can take them to the Gucci store to have them tightened by a professional worker.
Well, as far as I can see, the Gucci sunglasses you bought is not quite suitable to your face size. Therefore, it would be wise for you to ask the seller for replacement, generally speaking, there would be a little bit extra transition fees awaiting for you. However, even if you manage to tighten it for a short time, as time goes by, you cannot enjoy comfortable use of the glasses.