To maximize your benefit from spending gems, I recommend that you save most of them for purchasing store upgrades.
Buying GainsWeek tickets is sometimes necessary, I always keep 700 gems available for that because the items are worth the investment. Continuing to play GW after getting all currently available prizes/emails won't help you nearly as much as upgrading your shop. Save any unused tickets for the next GW, when they'll provide value for you again.