If an aircraft is /Y, /C, or /I (RNAV no GNSS), can they fly an approach chart that says "DME/DME/IRU or GPS required"? GPS in my mind is a GNSS. If you don't have DME/DME/IRU and are RNAV (non-GNSS) capable, can you legally fly the approach?
An example of a chart that has the "DME/DME/IRU or GPS required" notation is here.