Just grab a couple of people and farm the lower/higher floors of Palace of the Dead and/or Heaven on High? Many of the items dropped by Hoard sacks still sell decently and the rarer Hoard items can even be over 100k on the MB depending on your server's economy... Plus, that Night Pegasus whistle is still selling for over 10 million gil on my server... Palace/Heaven are fast and easy gil for sure.

Likewise, other people suggested ventures for Allagan pieces, but you might have better luck doing quick ventures, which can turn up a lot of junk--but, if your retainers are decently geared--can also turn up rare glamour items and even expensive dyes. If you retainer brings back a set of jet black dye, for example, you can usually sell a set of two for between 250-500k, from an hour-long venture...

But basically if you're not saving for a house or sinking your gil on tons of glamour items and rare minions, you really shouldn't ever struggle for gil... The game provides you plenty for just completing basic tasks.