Even if you are worried about the middle, a mat wouldnt do anything. In fact the only thing it would do is increase the risk of applying pressure to the bottom glass, which you dont want.Yes it has black trim around the bottom
I’m worried about in the middle it has a gap the ends are all ok it’s just the middle
Ideally a rimmed aquarium should be supported all the way around the rim, with all 4 sides touching and level. Worst case all 4 corners need to be supported and level for standard tanks under 4 feet, which means some little gaps in the middle should not be an issue. I've seen people put 6 foot tanks like this but I wouldn't recommend it.
Have you filled the tank with water to see if the gaps are still there? Is your stand top flat/coplaner/level? A lot of times the tank will settle after filling. How big of a gap are we talking, like a couple mm? Are they on one side or two? Short sides or long sides.
This video may help: