Does it require negative _mass_ or negative space-time compression? We know gravity waves exist, so space time can oscillate between 0 and N where N is positiv

Does it require negative _mass_ or negative space-time compression? We know gravity waves exist, so space time can oscillate between 0 and N where N is positive right? Are we so sure there is not a -N < 0 < N? Not even close to an expert in this stuff.