Here is what I know.
GMU 371 is comprised entirely of the Yakima Training Center.
Access to, and exit from YTC is through one gate only which is near Selah.
You must show proof of insurance and vehicle registration to enter.
You must have a valid recreation card to hunt. The cost is $20 and is valid for one year.
You must register every firearm you wish to take on the training center.
You may not camp overnight.
The training center is comprised of 16 Training Areas plus some other zones such as the DMZ. The open/closed training centers change each day. You cannot know what will be open the next day until approximately 1700 hours the day before.
You must check in at Downrange Control each time you access the training center.
You must specify which training areas you wish to visit. You may designate up to three. If you wish to hunt a training area that you did not specify, you must return to Range Control and change your designation.
You may not enter more than two hours before sunrise and must be off no more than two hours after sunset.
Some of the training areas may take up to an hour to drive to. Most of the roads are somewhat maintained but expect to get a flat tire.
There are not a lot of elk on the training center, but there usually are a few.
The training center is vast: about 500 square miles.