After 30 years of being in this line of work, I can confidently recommend the Larsen brand products. While you can find less expensive antennas and mounts, you will not beat the quality and value. I'm running 20+ year old Larsen antennas, I use them almost exclusively for personal and work use. I've never had one let me down.
There are also other really good brands, Laird, MaxRad, etc.
Different radios have different antenna connectors, so you'll need to pick a radio to know which one you need.
I'd recommend this mount:
https://www.theantennafarm.com/catalog/larsen-nmommrfme-4523 $29.95
I'd recommend this antenna:
https://www.theantennafarm.com/catalog/laird-technologies-qw450-797 $6.95
It's already tuned for the GMRS frequencies, so all you need to do is install it. No cutting or tuning required.
You'll need one of these adapters to go between the magnetic mount cable and your radio:
https://www.theantennafarm.com/catalog/adapters-304/fme-adapters-307/fme-male-adapters-320/
A quarter wave antenna, like I linked to above, will give you good all around performance. There are other higher gain antennas you can use, but higher gain antennas come at a trade off. Try the $6.95 quarter wave antenna first and see how it works. You'll always have a use for it, so it won't be lost money if you decide to go with something else later on.
I'd also recommend steering clear of the Chinese knock off antennas. Tram, Browning, and a few others are lower cost, lower quality replicas.
Also, stay away from the amateur/consumer grade antennas. Most of them are not designed for GMRS frequencies, and no matter what they claim, they have no magic to them that makes them work better than the commercial antennas.
A good basic antenna and mount will provide a lot of options at a low cost. Sticking with the NMO mounts will let you choose from the widest selection of antennas down the road.