The GP300 5-Tone (aka. Select-V, Select-5, etc.) radios look like a normal GP300, but they aren't. They use a different controller in the radio which requires different RSS. This RSS does not store the codeplug information in the "normal" format, but instead saves the file as an S-record.
In order to change the number of channels in the radio, you will need a utility called GP-Tools. This may also have various names such as GPUT, GP-Utility, etc. This utility allows you to edit the codeplug of the radio directly.
WARNING: Using this utility to edit the radio directly is VERY dangerous and leaves the possibility of creating yourself a paperweight that looks like a GP300. Proceed only if you are familiar with hex editing and are confident that you know what you are doing. If the bytes you need to change don't match what is listed, be very cautious about proceeding, you may be editing the wrong byte!
Ok, now for the procedure... Fire up the utility program and connect your radio and programming hardware. Read the radio and use the edit function to do the following:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
0000 == == == == == == == == == == 00 == 17 00 00 03
0010 01 95 3A 3A 3A 3A 3A 3A 3A 3A 7A 7A 7A 7A 7A 7A
0020 7A 7A 06 06 06 06 06 06 06 06 38 02 2D 1D 1A 18
The above example is for a VHF 25kHz radio. The example codeplug could be different in some fields from yours, because this example is a default codeplug for a 146-174MHZ radio. It will work with all frequency ranges (UHF radios too).
Edit only the == marked fields!
Referring to the above codeplug disaply, you should find that offset 0x0000 through 0x0009 are the serial number (in hex) for the radio, and offset 0x000B is the max. channel field.
For editing the serial number, convert your ASCII characters that you want for a serial number to hex and enter them into the serial number area.
To expand the maximum number of channels from 8 to 16, change offset 0x000B to the corresponding value in the table below.
VHF 12.5kHz: VHF 25kHz: for 8 channels:0D for 8 channels:19 for 16 channels:07 for 16 channels:02 UHF/low 12.5kHz: UHF/low 25kHz: for 8 channels:0E for 8 channels:1A for 16 channels:05 for 16 channels:06 UHF/high 12.5kHz: UHF/high 25kHz: for 8 channels:0F for 8 channels:1B for 16 channels:04 for 16 channels:03