Thank you for your responses.
Nologo, I use lossy (mp3, m4a) files. Gapless playback works fine with them on my desktop computer (e.g., iTunes, foobar2000 on Windows, and maybe Strawberry on Ubuntu) and my old phone (Music app on iPhone).
Lucario, my old post linked above describes the problem in greater detail, but basically if I keep gapless playback enabled, each transition from a track to the next track leaves the new one playing silent, and I need to interact with the playback in some way (e.g., rewind) for the sound to come back.
It's interesting that no one else has run into this. Given it's occurred with both Metro and Vanilla Music on my GOS phone, and I read online this could be a problem with custom OS's, I figured the apps' interaction with GOS could be the problem. However, the data I get from at least you guys not reporting the issue or hearing of it (unless everyone is using lossless as Lucario hypothesized could be a solution) suggests I should be able to get it to work. I will keep digging.