Pretty sure I haven't unlocked all the courses yet. I beat all the challengers and now I unlocked.. What was it. Like rivals or some shit? Then I stopped playing.
Yeah, it's the second round of versus mode. It will unlock extra stuff, like you can get a faster golf cart, more fishing spots, etc. from beating all of them.
There should only be five courses to unlock in the game, one came free with preorders, and the rest you purchase. You should have Eagle City, Alpine, Kanaloa Beach, Vortex Valley, and Imperial Garden. The free one with preorders was Green Country Club. After that I bought Balata, Nam Rong Bay, and Pine Hills...
The easiest way to rack up gems is to go to each course in the open world online, and each 9 hole area has one hole with a ton of coins, warp tokens, special golf balls, a style, and gems laying around. Each nine holes should get you between 2-4 gems. Just haul around and pick them all up, usually get 30'ish gems in 30 minutes or so.
I usually use the Grass Cutter ball so if you end up in the rough it doesnt affect your shot nearly as much.