From what I see, I’m guessing your jeans have a large front button that is pushing out the buckle therefore creating a visible gap. I’ve had a similar issue with certain pants that have larger / rounded buttons.
Mine sits like that too. I think this is because of the design of the buckle. With traditional buckles, the belt fits snug all around whereas with this Gucci belt, I have had to move it around a bit so it sits right. Don't worry too much, I doubt other people will notice the tiny gaps. Enjoy your lovely belt