Motorola GP-300 comes in several different model numbers. The best model number for use on the 2-meter ham band and the public safety band only is set by the factory to only TX down to 146.000.
To enter a frequency below 146.000, just hold down the shift key, and enter your frequency that way.
Example- 145.170 would display !$%.!&). When you press tab to go to the next field in the mode setup, the correct frequency will be displayed.
I have successfully programmed my radio down to 136 MHz, TX/RX. This also works some other Motorola radios, I have used it on a M-1225 with success, and a UHF GP-300 also with success.
You will just have to try it on yours.