Thanks a lot everybody !

Thank you all for the clarifying information! The problem has been 
solved using the command recommended by Pablo. All my attempts to clear 
the Windows font cache have failed, except for this one:

  mtxrun --script cache --erase && mtxrun --generate

I started by copying the fonts into the \texmf-fontes\data\ directory, 
then I emptied the cache. Then, the following command:

  mtxrun --script fonts --list --pattern=didot --all

works and correctly returns otf and ttf theano and gfs fonts.

And thanks to Andres : your MWE compiles perfectly !

Now, I must confess that I don't know why installing new fonts in 
c:\windows\fonts, even after laboriously purging the Windows font cache 
and reloading the CTX font cache, didn't have the expected effects ...

Le 24/09/2023 à 07:50, Andres Conrado Montoya a écrit :
> I did a test with Windows 10.
>
> Installed a fresh copy from the garden, Got the GFS Didot font from 
> Google Fonts, and installed it system-wide (C:\Windows\fonts). After 
> that, I ran
>
> mtxrun --script fonts --reload
>
> Then tried the pattern suggested but did not work. However, then I 
> tried a pattern without asterisks:
>
> mtxrun --script fonts --list --pattern=didot --all
>
> This returned the font:
>
> C:\griego>mtxrun --script fonts --list --pattern=didot --all
> identifier           familyname    fontname filename                   
>                 subfont instances
>
> gfsdidot             gfsdidot      gfsdidotregular 
>  c:/windows/fonts/GFSDidot-Regular.ttf
> gfsdidotnormal       gfsdidot      gfsdidotregular 
>  c:/windows/fonts/GFSDidot-Regular.ttf
> gfsdidotregular      gfsdidot      gfsdidotregular 
>  c:/windows/fonts/GFSDidot-Regular.ttf
> theanodidot          theanodidot   theanodidotregular 
> c:/windows/fonts/TheanoDidot-Regular.ttf
> theanodidotnormal    theanodidot   theanodidotregular 
> c:/windows/fonts/TheanoDidot-Regular.ttf
> theanodidotregular   theanodidot   theanodidotregular 
> c:/windows/fonts/TheanoDidot-Regular.ttf
>
> trying to render Pablo's example didn't work at first, but after i 
> called the fonts with the name returned by mtxrun:
>
> \mainlanguage[agr]
> \definefontfamily[mainface][rm][gfsdidot]
> \definefontfamily[mainface][ss][theanodidot]
> \setupbodyfont[mainface]
> \starttext
> GFS Didot:
>
> \doloopoverlist{\tf, \it, \bf, \bi}
>   {\recursestring\input aristotle-grc\par}
>
> \ss
> Theano Didot:
>
> \input aristotle-grc
> \stoptext
>
> Everything compiles correctly, and the font is used. Since it's only a 
> single font file with no variants, you don´t get italics, bold etc. 
> But the font is being used.
>
> -- 
> Andrés Conrado Montoya
> Andi Kú
> andresconrado@gmail.com
> http://sesentaycuatro.com
> http://messier87.com
> http://chiquitico.org
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> ----------------------------------------
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Ancients&Moderns
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