If you want greyscale backdrops, add/edit the follow entry to your ~/.config/gmpc/gmpc.cfg

1
2
3

[Backdrop]
alterations="6"
[]

0 = no alteration.
1 = add casing. (you do not want this for backdrop)
2 = darken
4 = decolor.

you can do darken, then decolor darken+decolor = 6