I obviously don't build LOS, but I've never had a problem with Gapps showing up when sideloading LOS via ADB. I have always used @tdm instructions, as mentioned by @EskeRahn. I adb sideload the latest LOS nightly, then reboot to recovery, which changes you from the a/b partition you were on to the other a/b partion where the update was installed. Then I adb sideload Magisk and Gapps (in that order, if it makes a difference-- I don't think it does).