I think it means that its valid on Metrolink in City zone only (IE Victoria to Deansgate/Castlefield or Piccadilly, as appropriate).
I am basing this on journeys like Heald Green to Walkden, or Hazel Grove to Rochdale, where this is the case
I think your supposed to take the Northern Manchester Piccadilly - Chester via Altrincham service.
'Rte Metrolink (GM)' does indeed mean tram in the City Centre only, not the line from Deansgate to Altrincham.
I'm not going to suggest trying the tram beyond the City Centre, partly because that would be wrong, but also because if a Metrolink RPI did give a monkeys, I think the 'Penalty fare' (or is it 'standard fare' these days) is £100.