Perfectly gapless. Using Jukebox 2112 decode-to-wav (or use JB yourself to listen to the .opus pair; turn DSP off). I don't hear a problem in the transition from b1 to b2.
The attached zip has the original b1.b2.opus and the decoded .wav files. I listened to the .opus pair twice, and the .wav pair three times. Perfectly gapless.