Is it possible to access the field "totaltracks"/"tracktotal" in the layout files ?

Sorry, not currently.
In fact there is currently no support for it at all in gmb. I'm not sure I'll support the total_track info in the file tag as I never saw it as very useful. I think I meant to add a total_track field that would be deduced from the number of tracks in the album (and maybe a last_track field for the last track number in the album) but I guess I forgot as I haven't seen lots of demand for it.