A quick followup: Mark "Spookshow" of the N Scale Locomotive Encyclopedia, who's forgotten more about N Scale locos than I'll ever know, commented "on another network" that there's an easy way to tell the difference between the DCC ready GP20s, which were produced in 2011, and all the other ones which were produced from 1999 until then:
The July 2014 issue of Model Railroader, Page 60, has a step by step article on how to install a drop in Digitrax decoder into a Walthers GP20, formerly Life-Like.
It should be noted that not all production runs of the GP20 have the light board version of the chassis that is shown in the article as being a swap-out for the Digitrax DN163LOA decoder board. Previous production runs have the small light boards at either end of the chassis, which to me suggests the TCS "split board" decoder installs (CN or CN-GP).
Of course we're not privy to actual production numbers but I suspect that the majority of the N Scale Life-Like and Walthers GP20s are of previous production runs. It would be wise to check your particular Geep before buying a decoder.
George Irwin, Rochester NY