I believe playing pretty much anything via the Wiim home app (wha) is gapless, ditto for bubble on android.
I'll caveat that by saying my experience is reasonably limited from the plethora of services available.
Tidal, qobuz, deezer, amazon, soundcloud, my nas files - all worked fine iirc from wha, but that's not chromecast!
Tidal, qobuz, my nas files - all fine from bubble. Again, not chromecast.
Spotify connect, Tidal connect - fine.
Amazon alexacast (different to chromecast) - fine.
Also, we'd certainly hope, and expect, qobuz connect to be gapless when it arrives. When, not if
I've not used anything that works gapless with chromecast.
Yes, roon does, and seemingly some other things that create a single stream to chromecast.
Edit: to answer (or, rather, not answer) your question about apple music - I don't know what other options there are for streaming it to a wiim device that might give you gapless playback.