There are adapotor pins for Lionel to GG. I have used GG to o-27, but the GG is a taller tack.
I have not noticed that Gargraves track is higher than 027 track, at least no noticeable differance that ffects train operations.
O gauge track is higher than Gargraves and will need to be shimmed a little to use a Gargraves switch with 031 or larger track.
Lee F.