\startMPpage
      picture tt ; tt := lmt_outline [
          kind = "path",
          text = "\definedfont[name:texgyrepagellabold*default]foo f o o",
      ] ;

      fill
         for i within tt : pathpart i && endfor cycle
         withshademethod "linear"
         withshadedirection down
         withshadecolors (red, blue) ;
\stopMPpage

I'll add this:

\startMPpage
     draw lmt_outline [
         kind = "outline",
         text = "\definedfont[name:texgyrepagellabold*default]foo f o o",
     ]
         withshademethod "linear"
         withshadedirection down
         withshadecolors (red, blue)
     ;
\stopMPpage

so a single path option (no picture)

Hans