My penny is on GSt lacking or compiled on another Gnome version. The slackware diff, is too small to probably have any effect.
But also make sure "liboil" is within. Preferable it should been compiled on same Gnome as the GSt or atleast the liboil should fit the running GSt.
I mention this, coz I did gnome on Slax and I separated GStreamer from the "gnome-desktop" package. Therefore I often forgot to put it back into also my own modules folder and I have ran into this issue countless times.