I am going on the assumption that you mean GM-30, and because the GM-30 is a "GMRS" radio, you don't need to worry about the offset. You just find the correct repeater channel based on the frequency (probably CH25?), then add the transmit tone - don't worry about the receive tone for now because that is just more things to do wrong and confuckulate yourself with.
You know if you are connected by pressing the push to talk button and calling out and asking if anyone can hear you. If someone answers "YES", or, if after you let go of the PTT button you hear a short blip of static (known as the tail or 'kerchunk') then you did it right.