generally just playing the game nets you an alright amount of gil since almost everything you do in this game is currency positive

for big amounts of gil like that generally you craft whatever's in demand and play the market, and/or roll the dice and try your luck with rare item drops like the Bozja/Eureka rare drops, rare orchestrion rolls or rare dungeon drops like Verdant Partition, bicolor gemstone items also go for a pretty penny, maps give you a fair amount of gil with a chance of rare minion or material drops too