just flashing gapps does not solve the problems. google sync is broken, youtube is crashing,...To get TWRP working you'll need an older build (maybe even format data).
To get Gapps working you just need to flash gapps.
Of course, if you want TWRP to be able to mount data, you'll need to stay on an older release until TWRP gets updated (and this may take months).
flashing an older build is not possible...error E3003: cant't install this package