So, what you have is a 45 watt UHF analog radio. It'll transmit/receive in the 450MHz to 520MHz band.
It can be programmed to hold up to 128 channels. It will do analog LTR trunking.
While the radio can be programmed for GMRS frequencies, the radio does not have the correct FCC type acceptance for GMRS.
Since these are a professional grade radio, they don't come fully enabled like a consumer CB would. The radios come from the factory with no programming info in them. The radio shop that sells the radio would program the radio with the features needed by the end user, and the frequencies that were licensed to them by the FCC.
Sounds like the customer that purchased the radio didn't need much in the way of channels or features.
KMC-65M is the correct microphone for the radio. You can purchase them on line.
For the antenna, you will need an appropriate UHF antenna that has been tuned for the frequencies you are licensed to use. The type of antenna will depend on what your planned use is.
They are fairly modern radios, and still sold new by Kenwood. List price for a brand new one with the microphone, mounting bracket and power cable is around $450. Interesting that you found such a new radio at a garage sale with no accessories.
So, what do you do with it?
Well, as I said, it -could- be programmed for GMRS and it would work just fine. It is missing the FCC Part 95 type acceptance that the FCC requires for use on GMRS. That doesn't always stop people, though.
If you want to program it, you'll need Kenwood KPG-135DNK programming software and the KPG-46XM (or equivalent) programming cable.
Here's an issue, though.
Most GMRS users utilize wide band FM. This radio, since it's designed/marketed to commercial users, will not do wide band FM. It's limited to narrow band FM. That's fine, and it'll still work, it'll just sound like the audio is really low on a wide band radio. The other wide band radios may sound distorted when received on this radio.
You can contact Kenwood and get a Wide Band license for the software, that'll let you use the wide FM. But you'll need to provide Kenwood with your GMRS license and they may charge a small amount. Just make sure that the software you buy is a legit Kenwood original, and not a bootleg downloaded copy. They'll want your software activation code.
Good find, that's a pretty decent radio.