Not a definitive list, but some potential options.
Chromecast with Google TV
Plex for Android (TV) running on CCwGTV can be controlled from Plexamp and will be gapless, but AFAIK does not support multi-room grouping.
Sonos
I replaced a couple of CCAs with second-hand Sonos Connects around 6 months ago solely for gapless playback.
Note that Sonos will use rg_track_gain (if set), not rg_album_gain, for loudness leveling which can be almost as annoying as gaps for some albums, but this can be worked around and I’d be happy to provide more information if required.
Apple AirPlay
FM Transmitter
Suitability will obviously vary depending on the quality of your source files and audio components.
Patience
There are intentions to add controllability (receiver role) to Plexamp and provide a headless version which could run on RPi and be controllable from a mobile or desktop Plexamp.
