I was nearing the frustration point, so I decided to put up the clicker and treats for a while, and brushed out his tail instead. It was kind of funny--he sat there and whined and cried growled and complained the entire time, but made no attempt to get up or leave. It's not like I was holding him to the ground and brushing it; any effort on his part and he could have easily scampered to the far end of the play yard. I guess sitting there and grumbling about it was more fun.
I went back inside for about an hour or so, and came back out later with treats and the clicker in my pocket again. This time, though, I didn't start giving commands right off the bat. I just sat there and hung out with him with treats in my pocket for a while. He could smell them, and they were driving him crazy. Pretty soon, Gizmo was doing every single trick he knew (and other assorted acrobatics), pawing at the pocket, whining at me, etc. to try and get me to cough up a treat.
This time, when I started giving commands, he did every trick perfectly, instantly, and right the first time. It really does all come down to motivation.