I too am looking for GMRS base antenna. The wife is not particularly keen on antennas going all over the roof or anything super tall. I think I can get away with a small antenna mounted to the back side of my metal fake chimney. I am looking at something like a Tram-Browning BR-6140 or Sirio CX 455 as they are about as tall as I think I can get away with, at around 26" tall. Any thoughts?
Mounting it in close proximity to a metal chimney is going to make it directional away from the chimney.
26" tall antenna can work well if you are not expecting huge gain numbers. It'll be fine for a low level antenna and for nearby repeaters.
Personally not impressed with anything from Tram/Browning. Cheap Chinese junk antennas. It'll probably work fine if your budget is really tight, but cutting corners on the antenna system is a rookie mistake. The antenna system is the most important part. I'd rather have a $200 antenna and a $50 radio than a $50 antenna and a $200 radio….
I do have a Sirio 5/8 wave VHF antenna that came with AIS receiver. So far it's done just fine.
Also, pay close attention to your coaxial cable. At UHF, you need to carefully consider feedline losses. You need to take into account the length of the cable and the losses involved. Also, proper waterproofing of outdoor connections is important. Don't forget grounding a lightning suppressor.