Gapless playback is absolutely supported with Mosaic Control so there’s something else going on here. What format are the files in and what is their origin (purchased downloads or ripped CDs)?
There is a known issue with some flac files that have extremely large embedded artwork which causes a delay in playback start. If these are flac files then try converting a few of them to AIFF and see if gapless works.
Please note that no third-party control apps (MConnect, BubbleUPnP, etc) will achieve gapless playback with any of our products. This is due to the fact that the internal playback architecture isn’t actually UPnP so the method that these apps use to accomplish gapless playback doesn’t exist.