\definefontfamily [mainface] [rm] [texgyreadventor]
\definefontfamily [mainfacf] [rm] [gentium]
\setupbodyfont[mainface]
\switchtobodyfont[12pt]
\starttext \rm Tex Gyre Adventor\\
\switchtobodyfont[gentiummainfacf]
Tex Gyre Adventor
\stoptext
</context>