A google.com search finds multiple hits on the use of "AT"
commands to initialise the Gtran Dotsurfer, including a PDF
manual from the manufacturer.  But the manufacturer's own site,
www.gtranwireless.com, is not responding at the moment.

So I'm puzzled.  At the very least, I'd expect it to respond to
the most basic (do nothing) AT command

       AT

<carriage return> (but without a <linefeed>) with an response of
"OK".

Perhaps there's some extra step required to put the modem into
command mode.  If the modem's in data transfer mode, the usual
way of entering command mode is with the "escape sequence"
consisting of the three-character string

       +++

with a pause of a few seconds (the "guard time") either side.
To reenter data transfer mode from command mode, the command is

       ATO

where "O" is the letter, not the number (followed by <carriage
return> of course).

John