Do those use the Java applet?
If so and it's the same one as Custel 2.5 (not sure about the other ROMs):
1. Attempt to download the gmail applet from the Get Faster gmail link from http://gmail.google.com
2. After waiting for the Midlet Manager to complete, exit both the browser and Midlet Manager.
3. Open the Java applet and select the option on the Menu to Install from a local source (which should be there after it fails to download successfully from the google site; a .jad file sitting in My Documents).
I have the file on my phone. If you need it, please advise. Hopefully, this helps.