Thank you Wolfgang,
now it works partially (see the first attachment): it uses different
optical sizes, but not always the right ones.

\definefontfamily [default] [rm] [GaramondPremierPro] [opticalsize=yes]
\setupbodyfont[default,12pt]
\starttext
\dostepwiserecurse {6} {22} {2} 
  {\switchtobodyfont[#1pt]
   \purefontname\font: \input ward
   \blank}
\stoptext

My garamondpremierpro.lfg is a modified version of ebgaramond.lfg.
While I was waiting for an answer to my mail, I made a modified version
of type-imp-ebgaramond.mkiv: type-imp-garamondpremierpro.mkiv (second
attachment).
Then I tried this:

\input type-imp-garamondpremierpro.mkiv
\setupbodyfont[GaramondPremierPro,12pt]
\starttext
\dostepwiserecurse {6} {22} {2} 
  {\switchtobodyfont[#1pt]
   \purefontname\font: \input ward
   \blank}
\stoptext

This works as expected (see third attachment): "GaramondPremierPro-
Capt" under 8.5pt, then "GaramondPremierPro" until 13pt, then
"GaramondPremierPro-Subh" until 21.5pt, then "GaramondPremierPro-Disp".

How can I make it work with \definefontfamily?
I prefer its simpler syntax to typescripts like "type-imp-
garamondpremierpro.mkiv".
I tried this:

\definefontfamily [default] [rm] [GaramondPremierPro]
[opticalsize=yes,goodies=GaramondPremierPro]
\setupbodyfont[default,12pt]
\starttext
\dostepwiserecurse {6} {22} {2} 
  {\switchtobodyfont[#1pt]
   \purefontname\font: \input ward
   \blank}
\stoptext

but it does not work as expected.
The file garamondpremierpro.lfg gets loaded, but its contents are
ignored when choosing the right optical size.

How can I tell ConTeXt to use the optical sizes defined in my goodies
file with \definefontfamily and without using type-imp-
garamondpremierpro.mkiv typescripts?