My general assumption about contributors is that the labor they're investing provides some personal benefit to themselves. That could be either the value they'll receive from using the gem in their own projects, or as a portfolio showcase when selling themselves to clients or potential employers.
Since you didn't provide a link to your gem, we'll also assume you haven't built it yet. If you want complete strangers to help you, you'll most likely need to build it yourself until it's reached enough popularity that people are submitting issues and requests for feature enhancements.
