Depending what level you are you could also try doing Treasure Maps - they can net you some decent Gil (not uncommon for our FC to run a few of the lvl 80 group ones over a couple of hours and net over a million gil).

Gathering can be very lucrative as you pretty much sell anything you get that's not used in a quest (either MB, or if the MB prices are low just vendor it), plus at higher levels you get Treasure Maps that you can either use, or sell on MB (lvl 80 Group Maps can easily get you 50-100k each depending on your server). Once you hit Ishgard there's also the Firmament/Diadem and mats from there still sell very well.

You won't get a super "this item will sell in like 5 minutes for 100k+ gil" but a lot of smaller items do sell very well and this coupled with questing/roulettes could easily get you your 500K in a matter of days.