If I am seeing this correctly in the product description, the gxt 500 radios are FRS/GMRS units. If so, they don't have a provision for replacing the antenna (and modifying them to do so would void their FCC authorization).
There are three major factors that affect range:
* Radiated power
* Receive Sensitivity
. (both of the above can be affected by antenna selection)
* Antenna height above surrounding terrain
With any decent transceiver, this third factor is far and away the most important. We test all of our transceivers (and those of our competitors) to confirm that they can operate out to the rated range. This test is done in a mixed-terrain location that has a 20-mile line-of-sight.
If you don't have line-of-sight (that is, if there are any hills, buildings, trees, etc), then your range will be constrained. Simply changing the on-unit antenna to another on-unit antenna might help slightly, but any significant improvement will be achieved by increasing your elevation (hold the radio at arm's length over your head, climb a tree, jump up and down and only talk at the top of your jump
).