It does UPnP gapless - so it’s totally dependent on the UPnP renderer implementation. I verified Lumin hardware gapless, as well as volume control, work correctly with this app.
In the unlikely event there are Lumin users here who are not using Roon but want to try JPlay app, please contact me to get a beta firmware and state your Lumin model.
Search seems to default to streaming every time which is very annoying. It doesn’t have a global overall search just a combined one for you library which you can’t seem to set as the default. Also seems to be missing most of my album covers where all other apps using same UPnP server have them.