I haven't experienced this problem with gel pens in flight or after a flight. Maybe try switching to a different brand or model of gel pen? The few times I've carried a Uniball Signo 207 gel pen it's worked fine on board and later.
If you choose a fountain pen, try to keep it with the nib pointing up during the flight - that minimizes the chances of pressure changes forcing the ink out and creating a mess. (Other tips include keeping the pen either completely full or completely empty of ink.)
As others have remarked, ordinary ballpoint pens work fine. I usually carry an old-fashioned Pilot V5 rollerball pen and can testify I've never had a problem with ink flow or leaks any time (even though the company's web site suggests that it's really not leakproof in flight). The "RT" retractable models of rollerball pens from Pilot are guaranteed to be flight-safe.