Wrtl923

The DB25-G IS on the CHIRP Support list.

The mere name indicates that it is optimized for Amateur 2m and 70cm bands. MURS is at 151 [channels 1-3] & 154 [channels 4-5], and GMRS is at 462 [1-7, 15-22 simplex], 467 [8-14, 15-22 Repeater/aka 23-30]

Both of the below I've used with good results:
Baofeng Magnetic Car Vehicle Mounted Antenna UHF VHF Dual Band
 
Walkie Talkie Antenna for Baofeng Antenna Upgrade 15.6-Inch Whip Dual Band UV VHF/UHF 144/430Mhz

If you're looking for an easy starting point to start testing, a mag mount may be a good bet...nmo is very common for the antenna connector, and you'll have no problem finding a variety of sizes of antennas to match up to it. Lip mount is another very flexible option.
In terms of antennas, anything rated for 450-470 or 460-470 mhz should work fine. One consideration to ponder is overall height: desired mounting point and antenna length. Generally more gain means longer antenna, and also means less vertical height of your signal. 1/4 wave antennas are around 6" long but depend on a bit of metal around the mount to work properly (about a 12" diameter circle). 5/8 wave run around 12" and are a bit less dependent, you can also find 1/2 wave that don't need a ground plane. Midland's 6db whip isn't a bad choice, 2 5/8 waves, but also close to 3 ft long. (Imo, a 1/2 or 5/8 wave, sometimes labeled as 3db gain, would be good starting point, since it's reasonably short but still has some gain).
At the radio end, most mounts will have a pl259 (the male side, or PLug) connector on the radio end of the cable (also known as uhf connector); this mates to a so239 (female side, or SOcket) on a mobile radio. For your use you'll want an additional pigtail with so239 to either sma-m or sma-f. If the antenna on your uv9g threads into the radio body, the radio is the sma-m, and the antenna is sma-f (so get so239 to sma-f). If the antenna screws onto a connector that sticks out of the radio body, the radio side is sma-f, and you need so239 to sma-m).

It's pretty common to do one or more of the following:
Upgrade your HT antenna to another, more adequate HT antenna such as a Nagoya NA-771G. This is a 15" antenna, and claims to produce 4.71 dBi gain. Still portable, but also long enough that you can't exactly be inconspicuous when using it. Get an adapter (so239 to sma-f or so239 to sma-m depending on your radio), and then connect the HT to a mobile or base-station antenna. As an example, I can connect my UV-5G via an adapter to an NMO magnetic mount, and then screw in whatever antenna I want, so long as it matches to NMO. I own a couple of options, the Midland 3db Ghost antenna and the Midland 6db 31" whip antenna. But with an adapter you can really mate it up to any GMRS antenna as long as one end is SO239. The second approach is far less convenient for "handheld" use. It would be practical for using the handheld as a base-station radio, or in a vehicle. Not as practical for walking around on a hike.