Getting back to the OP, I recommend activating "Tutor" mode in GnuBg. That way, the computer will interrupt you when it thinks you have made a significant error. I often practice this way against GnuBg. Because my errors are usually flagged and corrected, my win rate goes up.
The threshold I use is 0.02. If my errors are less than this, the GnuBg tutor does not correct me. For larger errors, it does.
Many times, of course, GnuBg is wrong. That's because I have the tutor set to use GnuBg's "grandmaster" setting for checker and cube decisions. ("World class" would also be a good setting.) A rollout will occasionally overturn a recommendation made by the tutor. The truncated rollout I use when my intuition tells me to check the tutor is described
here. It runs very quickly.
I have not kept records, but with an assist from the tutor, I think I win slightly more often than I lose against GnuBg. That's because of the short rollouts I make in the middle of a game. When I think my move is better than the tutor's, I roll it out. The end result is that I play about as good as GnuBg when I follow the tutor, and I play better when I add in the plays that I have rolled out.