Why can't I FaceTime to a Gmail account from my iPhone?
Your daughter needs to make sure her gmail.com address is associated with her iCloud account. From an iPad or iPhone, can you send her an iMessage at that same Gmail account (i.e. does the conversation turn "blue" instead of green)?

Email addresses can be added to FaceTime through iOS Settings or through the preferences in FaceTime.app on a Mac.

People FaceTime me all the time at my Google Apps account, so it's possible.