there aren't really "Standards" for 400-series highways, per-se. The 400 still has an at-grade intersection on it, and there are tons of sub-standard interchanges, tight curves, reduced speed limits on 400-series highways.
The general standard is that it should be a free-flow freeway, which the Gardiner and DVP meet.
The province may just retain the Gardiner / DVP name as well, of course. The QEW historically did end at the Humber River however, and when it was downloaded east of the 427 in the 1990's the City extended the Gardiner Expressway designation out to the 427.
As to why the Gardiner was built as an elevated expressway and not a tunnel in the 1960's.. we may need a reminder as to what the area the Gardiner ran through at the time. It was built through what effectively amounted to a heavily industrial area far from the city centre.