Are you referring to the opaque (white) glass on the meter or the entry pupil of the viewfinder (viewfinder eyepiece)? Neither should be loose, but the ring around the entry pupil of the viewfinder can be unscrewed for insertion of a correction lens for diopter adjustment. If this ring (also called eyepice frame in the instruction booklet) is loose, you can just screw it back in. There is no gasket.
You can find instructions both for the 6x7 system and the pentaprism viewfinder on butkus.org:
Pentax 6X7 instruction manual, Pentax 6x7 prism finder, user manual, PDF manual, free manuals