Here you go:
magick "object.tif" -write MPR:orig -alpha extract \
\( -size 3202x512 gradient:white-black -geometry +0+2212 \) -compose multiply -composite \
\( -size 512x2724 -define gradient:direction=East gradient:white-black -geometry +2690+0 \) -compose multiply -composite \
MPR:orig +swap -compose copyopacity -composite "object_masked.tif"
Based on this answer: https://stackoverflow.com/a/40383400/4199855