There is no direct integration with Xebra & Google mail. We recommend using a program called Affixa. http://www.affixa.com/
There is an option in the setup of Affixa that allows you to set it as your default. It seems this worked better than trying to set the default through the PC itself. All checkboxes under Email Settings in the Xebra client should be left unchecked.
When initiating an email from Xebra, it does take Affixa several seconds to prep and load the email and it seems it works best when you already have Gmail open in your web browser. Also, during the loading or "creating message" phase, the status bar will go to 100% but hang there for a second or two before completing. Other than that, it seems to work fine; carrying forward the proper email address for the recipient, default Subject Line and Message, and all associated attachments (both documents and file attachments).