i follow the dirxn fine, but neither the garage nor my community gate
clicker will program on the yukon -  same problem in my mom's pontiac.

I have certainly had my own challenges with openers.  Since you are
pretty sure you are following directions, I would just say:
1.  I think the button on the opener and the vehicle must be pushed simultaneously.
2.  Be sure both units are close enough to get the proper signal - I
had to park the car directly under the opener.
3.  My vehicle manual mentioned something about clearing preset codes
on the vehicle - I think I had to push two buttons in the vehicle for
a period of time.  If you are only following the info in the opener
manual, try your vehicle manual too.
4. Call the manufacturer of your opener.  They probably have
encountered the problem before.

thank you for the comment.  i did try all of the above, except to call
gmc, which i was trying to avoid, but i have a feeling it will be
necessary to invoke them.  i seem to have this problem with almost
every car i buy. now and then i get a surprise and the buttons work.