At first characters level up gives out good enough rewards, its after that that its really hard to get enough gems
anoymous
This not really a f2p friendly game, despite of how often konami handed out gems. Mainly due to how rapid the pack expansion pace is. You can never be able to open every pack in every box without cash. Just hope that your luck is good so you can pull the URs or SRs that are actually good instead of useless piece of shit like that fucking Judgeman.......
But to answer your question, without using money, gems can be obtained from:
-stage level
-character lv up
-konami's generosity
-tapping here and there
-battling legendary duelist (if you're lucky you can get 10 gems per chest, but in their sane mind wanted 10 gems when you could get SR cards from the LDs)
-farming drop skills from legendary duelist (dropped skills that your character already have will be converted to 15 gems, but yeah, SR cards are better)
anoymous
