Oh YES, he is right
USB is a master/slave or host/slave communication. Suppose you are connecting your GSM USB modem to the PC’s USB port. Here the PC is the host (and always will be) and the modem is the slave. Host initiate the communication by sending some data (requesting about hardware details) into the slave and the slave should reply to it properly before the actual communication begins. Thats how the host decide what kind of driver should be loaded to communicate with that particular slave.
In all USB devices that is meant to be communicated with a PC have a hardware inside it called SIE (Serial Interface Engine). It manages all the communication between PC and the USB device. So in order to communicate with such a USB device you should have drivers written for it.
In your case your simple microcontroller should act like a master and the modem as slave. So you can connect the modem only if you can code for a driver in your mcu
It is like,
|
GSM USB |
modem <— connecting circuits
> | USB driver
(slave) | code
|
| MCU
|
For more details about USB host/slave communication find this useful link
http://www.maxim-ic.com/app-notes/index.mvp/id/3803