GAPPs is "Google Apps", of which iirc the app Google Play Services is a part of.

MicroG to my knowledge is a reimplementation of Google Play's services (not just the app but the services it provides), but by default privacy-oriented, not tracking the user unlike the original apps, and you can choose which parts of the services ("APIs") MicroG uses from Google. Some trial and error for enabling/disabling services may be needed for apps to work though.

To remove Google Play Services, if you installed it as a system app, I'm not familiar with the process but I'd presume you'd have to remove it through the bootloader, assuming also you keep your phone unlocked as you mentioned you use LineageOS. If you installed it as an app, you can uninstall it as any other app, though from experience, some apps will misbehave for a few days, like sending notifications about not finding Google Play Services, Whatsapp insisting on making a Google Drive backup, etc., though those stopped after a while.