Free-to-Play EditAll of Habitica's gaming features are available in a "free-to-play" format. Although subscriptions and gem purchases generate revenue for the game, there is no "pay to win" scenario. "Pay-to-win" is described as being able to gain a significant advantage over those playing for free. With the exception of quests, gem purchasable items give no advantage to in-game stats and do not affect the completion of Dailies and To-Dos, which is how users advance in the game. This would defeat the purpose of the game, which is to motivate users to improve in their real lives. They are mainly cosmetic and aesthetic or, as Tyler Renelle calls them, "avatar flair" (e.g., backgrounds, hairstyles, and gear).
Many quests, including the quests which give Legendary Equipment and large gold and XP rewards, do not require gems to purchase. Quests requiring gem purchase, on the other hand, give smaller gold and XP rewards, along with pet eggs, which are purely cosmetic. Even those Quests which can only be purchased with gems can be shared by several players, only one of whom needs to buy the quest.
Support Habitica Edit
That being said, gems do perform a very important role in Habitica. By purchasing gems, users help to support Habitica and all of the work invested into making it great. Without the revenue gained through this and other methods of support, including subscriptions, Habitica would not be able to continue offering nearly all of its features for free. The game is free to play but it is not free to run.
As of November 11, 2015, players can also support Habitica by purchasing merchandise. Items include T-shirts and stickers displaying the Habitica logo.