You've only been playing a short while. Gil just comes naturally the longer you play, the more you do. The challenge log alone usually throws a few piles of gil at everyone every week. Some of the daily roulettes and grinding dungeons result in a trickle of gil. The tomestones that are rewarded are also often just traded to items to sell for more gil when players don't need anything anymore. The further you get into the game, the more opportunities there are.

Personally, probably the easiest source at 70 is unlocking Eureka. Lockboxes dropped from all the bosses you'll be fighting will have more materia than you'll ever need.

Additionally, for your ilvl boost, you'd be better served heading back to Mor Dhona to pick up the i120 Ironworks gear sold for Poetics tomestones. These can be further upgraded to i130 if you have a pressing need for it as well. After 50, this will be the common thing until you reach end game. Every ten levels, at the end of each expansion's content, you'll be able to get the best gear for that expansion's level cap by trading in your Poetics. It's gear that will mostly serve you through at least half of the next expansion.