Hey, hi.
I’d like to ask what would be the best way to upgrade gems. I have 615 of resonance and it has been so difficult for me to upgrade my gems.
Do I need to spend (real life) money to buy platinum and then buy gems?
What about the legendary gems? 
Thanks a lot 
The answer comes down to a choice of…
If you choose the first option, you’ll need to do things that earn your hero platinum, so that you can buy legendary gems from the Marketplace (you’ll require spares of the gems you want to upgrade, and also gems you can turn into gem power). Easiest way to do that is to ensure that you’re doing all 21 of your Hidden Lairs every week. 21 lairs, 2 layers per lair, 3 events per layer = 126 non-legendary gems, half of which are unbound and saleable on the Marketplace. Also, when you do content in a 4-man party (e.g. bounties, dungeons, elders, lairs) you can earn 12 non-legendary gems per day. You sell these on the marketplace to earn plat, then buy the legendary gems you want from the marketplace. This is slow. Depending on your resonance goal, this could take multiple months, to a couple of years.
If you choose the 2nd option, you can spend money on legendary / eternal legendary crests, then run Elder Rifts with them and hope you get lucky with which legendary gems you get. Either sell the ones you don’t want on the marketplace or use them for gem power to upgrade the gems you do want.
There’s basically no avoiding that it’s either your time or your money.
If you want, why not if you don’t want to, then the only answer is to keep playing by doing all those limited-time events and your daily tasks, which means pretty time-sink.
I played since the launch and my reso is pretty around 2k and got a few Rank 6 gems at best, but it doesn’t bother me that much since it was good enough to clear all PVE contents and allow me to get into the Top 20 in BG leaderboard.