\definecolor [transparent] [a=multiply, t=1, s=1]

\startuseMPgraphic{trans}
  fill OverlayBox
    withshademethod "linear" withshadevector (3, 0)
    %% (white, "transparent") doesn't work here
    withshadecolors (white, white) withtransparency (0, .7);
\stopuseMPgraphic

%% to illustrate the effect, actual background is white
\setupbackgrounds [page] [background=color, backgroundcolor=lightgray]

\defineoverlay [gradient-mp]  [\useMPgraphic{trans}]
%% requires external file gradient-t.pdf
\defineoverlay [gradient-img] [{\externalfigure[gradient-t][width=\overlaywidth,height=\overlayheight]}]

\starttext
  using external image:
  \startframedtext [background={foreground, gradient-img}]
    \input knuth
  \stopframedtext
  metapost:
  \startframedtext [background={foreground, gradient-mp}]
    \input knuth
  \stopframedtext
\stoptext