I tried following the instructions here but it says to put my full e-mail adress as my username.The thing is the e-mail setup thingy on my bb won't let me change the username.
You may want to contact your school's IT helpdesk. My school also uses the Gmail system, however they do not support the use of mobile devices to access their e-mail. I would check with them first.
If anything, you can set up the school Gmail address to forward the messages to your regular e-mail address. That's what I did and it works like a charm. The only downside is that if you send any e-mails out as a reply it will send from your regular e-mail, not the school e-mail.
Have you tried logging in via your PC to see if you can do it from their via your carrier's BIS site?
Based on your information, this should be the right link. Log in and see if you can do it from here. I was messing around on my BIS trying to set up my school e-mail to test it out, and it allowed me to change my username using the site. http://turkcell.blackberry.com/
You may want to contact your school's IT helpdesk. My school also uses the Gmail system, however they do not support the use of mobile devices to access their e-mail. I would check with them first.
If anything, you can set up the school Gmail address to forward the messages to your regular e-mail address. That's what I did and it works like a charm. The only downside is that if you send any e-mails out as a reply it will send from your regular e-mail, not the school e-mail.
All email programs that I know of allow different identities, so you can reply from any address. In Gmail > settings > accounts and imports > send mail as.