The non acceptability of GMAW for 321 cannot be argued from a mechanical, chemical, or metallurgical standpoint. Once the wire is melted it does not remember if it was on a spool or a stick. If you semi auto GTAW you will be using the same wire as GMAW. But the concern here may be the existence of drawing compound for the wire which would need to be cleansed for semi auto GTAW. However, it may be unnacceptable based upon customer specification, or if you insist on using 321 filler you will lose Ti across the arc, and therefore stabilization, in which case GTAW would be your only choice. This is why 347 has become the filler of choice.