Here is some information about removing it:
http://www.gmcmhphotos.com/photos/how-to-remove-wiring-duct-cover-plastic-trim/p9844.html
However it will probably crack.
I found that something called a Panduit electrical cable cover works great. The GMCs apparently varied. Some were 3" and some were larger.
My cable tray measures 3-3/16" with a caliper straight across the corners of the tray. Probably the same as what some have called a 3.25" tray. If you lay a tape measure across it the measure will bow a little and measure 3.25". The Panduit cover is 3-1/4" on the outside edges.
I found that I could hook the top of the cover over the top edge of the cable tray and then while pushing down on the top of the cover to bow the edge a little I could tap the bottom edge with a hammer and block of wood and get the cover to stretch a little so the the bottom edge would bow out and snap onto the lower edge. With it installed the top and bottom of the cover are at a small angle to the face (i.e. greater than 90 degrees at the edge).
It was a bit of work getting it on but it looks great. My GMC is a 1977 Kingsley.
You can find the Panduit covers at electrical supply houses. It comes in several colors.
I believe that Gene Fisher has information on his site under the Headliner Replacement section.
Emery Stora
77 Kingsley
Frederick, CO