I found this thread from the forum about a 68 GMC; look at 2nd photo; it's tough to make out and maybe you can PM him for a better one; assuming it is an original 250 GMC base truck.
http://www.67-72chevytrucks.com/vboa...light=1968+gmc
Here is a pic & listing from EBAY. Sellers seems unsure, as do we, of it's origin. This might be the correct emblem. The dimensions are correct for a fender emblem.
http://www.ebay.com/itm/50s-60s-70s-...ec6ec8&vxp=mtr
http://www.ebay.com/itm/50s-60s-70s-...ec6ca8&vxp=mtr
Perhaps someone with another 68 will confirm this.