Well, if you would enjoy it! It wouldn't disadvantage you per se. I'd say doing an A-level during a gap year, if it fits in well with your other plans, might be a way to keep up the practice of studying and might be a brain-saver if you end up earning money on the tills. Perhaps not something uber-academic if you've got lots else planned though.
A gap year is a good opportunity to get extended work experience, travel, earn money, just generally grow up I guess. So if studying pins you down too much, you may not be able to take that three-month trip across Asia (tho money may also be a factor in that too ... !). Generally, I don't see a problem if you're sensible with time, just make sure you plan what you're going to do.