Can you fix a fire door with a gap at the bottom?

Yes, nearly always, and it's seldom a big job.

Usually the door has dropped on its hinges, and new hinges with a rehang put it right. When the floor has been lowered instead, a threshold strip or a drop seal can close it up. Which one we use depends on the door and on what's underneath it.

One thing catches people out. Measure with the carpet down. A gap that looks fine over bare boards can turn out too big once the carpet is fitted. The limit depends on the door, and your inspection report gives the figure it was checked against.

Please don't plane the door to make it fit. That takes wood off the very part meant to hold fire back.

And if the door has panels, no repair will rescue it. We'd advise a plain certified doorset.