There are a few different ways.
1)Find your local Ma/com shop (may cost $$$)
2)Find a ham who can do it - plus there are often folks on eBay who advertise programming
3)Look for the "conventional" DOS programmer software (which is out there with GE/Ma-com permission (or by the conventional windows software, not too expensive - the software that supports trunking is NOT cheap), and buy either a "RIB" (Radio Inferface box) and cable from Price Industries, or get a "ribless cable", which makes sense if you are not going to re-flash the radio, or not doing multiple types of radios. I use a RIB because I own about 6 different kinds of GE radios that use the same RIB