Wirelessly posted (8700g: BlackBerry8700/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100)
Reply does not provide the option to edit the send using field. However, if you forward you will be able to do so. You also should have the gmail address option in send using when you compose. If this is not the case, try a hard reset. If that isn't the cure, make sure you actually have the gmail service books on the blackberry. If you do not, resend service books.