Yes as *G with miles&more ffp and eurobonus ffp flying on LH, LX, OS, SK in economy you can get access (+1guest) to the schengen menzies/servisairlounge. No need to get vouchers or stamps, just show up with your boardingpass and the ffp card. They'll note down name and ffp nr and you're admitted.

I do not know if other *G on other ffp will get access because it is not an official star alliance member lounge but a contract third party lounge. It might depend if they have a deal with the lounge operator.