Free-to-Play[]
Most of Habitica's task management and 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. That would defeat the purpose of the game, which is to motivate users to improve in their real lives. Although some may be demotivated by not being able to access quests only accessible by paid for time traveller hourglass or gem purchases (unlike gems, you cannot "earn hourglasses).
With the exception of quests, gem-purchasable items give no advantage to in-game stats and do not affect the completion of tasks, which is how players advance in the game. Although players will earn less in rewards due to not being able to access some later quests. With the exception of quests, gem-purchasable items are mainly cosmetic and aesthetic (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. Although pets are linked to later rewards. Even those quests which can only be purchased with gems can be shared by several players in a party, only one of whom needs to buy the quest.
Support Habitica[]
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 gems and 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.
Players can sometimes also support Habitica by purchasing merchandise such as T-shirts and stickers. There have also been two Kickstarters, which allowed players to choose to support Habitica.