Nothing is "bug tight" - even big ol cockroaches can essentially get into a paper thin gap - if your baseboards cover the hole...yeah your good imo, its really cosmetic. As you said the drywall ends at the sub floor with a little gap, unless you go caulk all of that, bugs will get through if they want to.
Perhaps some could say your going to have some flow with A/C negative pressure and you should patch the holes... if they did a consistent job - buy a sheet or 1/4 sheet of the proper drywall, cut 1" bigger square / rectangle- do the paper overlap method, and plop them in the holes with a bit of mud, and be done with it- then put up the base boards, again it does not have to be pretty if its not being seen.
By paper overlap patch - you cut a piece of drywall about 1-2 inch bigger than needed, then cut the "drywall inside" out from the overage / excess but leave the outer "paper" layer- put some drywall mud on the paper, plop it on, put a little more on the outside and give it a wipe with a trowel etc. and be done, doing it "better" would be some sanding and repaint.
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