Could this mean that if I Connect the GSM (3.3v) and connect a USB to UART it will provide 4.5v not 3.3v that could harm the GSM?
Yes, You can safely obtain up to 100 mA from the USB port and use it to power your device. In case you will need 3.3 V you have to use a voltage regulator to drop it from 5V to 3.3 V


.... Please give advice. I need the PC to see what I'm doing.
If you want to connect your system to the PC via RS232 port then you should use a level translator as MAX3223, which will convert RS232 level to 3.3 V TTL and vice-versa. Without it you will fry your modem input line.

Cheers

Al.