After many experiments we decided not to use MD300 USB modem with GadgetPC. Unfortunately this modem works badly with USB Full Speed Host port.
It works nice with USB High Speed Host port.
We tested both ports on a PC platform.
On Gadget PC we have a full speed port.
Thus we switched to modems from Sierra.
We tested Sierra Compass855 Aircard device. It worked very well.
http://www.sierrawireless.com/product/compass_885.aspxAlso, this modem is added to GadgetPC release.
So I guess you will not face any problems to configure and use that.
We tested the modem on AT&T network without authorization and with PAP authorization.
In both cases the modem worked reliable.
Please let me know if you need extra help to get started.