The ‘view season reward’ button literally has a gem in it. There is otherwise no point in grinding the crown gamemode other than for badges, and there is 0 way for free-to-play players to earn gems. Although gems is a premium currency, it should be available to all players as it provides great progression if items are bought in the shop. Personal suggested divisions are:
1st: 500 gems
Top 10: 300 gems
Top 50: 200
Top 100: 150
Top 500: 100
Top 1000: 50
Top 5000: 25 (or nothing)