I apologize if this has been asked before.
I have seen tanks that are loose, rock back and forth easily. Bolts are pretty snug. Appears the gasket is too tall and the bowl is not close to that lip on top of the bowl that the tank is supposed to rest on. Are their different height gaskets or do you snug the bolts down until tank rests on that lip? I'd be concerned with over tightening but I don't know.
If this is a new toilet, and not a replacement gasket that may not be the right size, then it should be able to be tightened down. The key to this is tighten a turn or two on one side, then do the other so you bring it down evenly. I like to put a piece of paper between the tank and the bowl and stop when I start to get friction...you should still be able to pull the paper out.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013