Let's assume you own the Gog collection : 
Install it.
I made a batch to simply do what's written here : 
http://www.jonhunt.c.../gogrt66grp.txt
You can get it there : 
https://pastebin.com/kGdsgH0q
All the necessary files will be in the folder temprt66 in C:\GOG Games\Redneck Rampage Collection\Redneck Rampage\
Go to temprt66 and edit with notepad / notepad ++ the USER66.CON file : 
Remove Route66\ on all those lines 
definelevelname 0 0 Route66\Start.map 03:50 01:49 HIT THE ROAD
definelevelname 0 1 Route66\Carnival.map 06:15 04:13 CARNIVAL OF TERROR
definelevelname 0 2 Route66\Truckstp.map 04:10 02:11 BIG BERTHA'S
definelevelname 0 3 Route66\Brewery.map 05:00 03:02 BIG BILLY'S BREWERY
definelevelname 0 4 Route66\Flea.map 05:15 03:15 FLEA MARKET
definelevelname 0 5 Route66\Slaughtr.map 03:10 01:03 SLAUGHTERHOUSE
definelevelname 0 6 Route66\Minigolf.map 06:50 04:49 FUN PARK
definelevelname 1 0 Route66\Gas.map 06:15 04:12 GASSIN' UP
definelevelname 1 1 Route66\Ranch.map 05:00 03:01 HOUSE OF ILL REPUTE
definelevelname 1 2 Route66\Coffin.map 04:45 02:32 MYSTERY DINO CAVE
definelevelname 1 3 Route66\Campy.map 07:50 05:53 CAMPY CRYSTAL LAKE
definelevelname 1 4 Route66\Bigfoot.map 04:00 02:02 BIGFOOT CONVENTION
definelevelname 1 5 Route66\Hoovrdam.map 10:00 08:13 HOOVER DAM
definelevelname 1 6 Route66\Alien.map 07:10 04:55 ODDITY MUSEUM
So you should have
definelevelname 0 0 Start.map 03:50 01:49 HIT THE ROAD
definelevelname 0 1 Carnival.map 06:15 04:13 CARNIVAL OF TERROR
definelevelname 0 2 Truckstp.map 04:10 02:11 BIG BERTHA'S
definelevelname 0 3 Brewery.map 05:00 03:02 BIG BILLY'S BREWERY
definelevelname 0 4 Flea.map 05:15 03:15 FLEA MARKET
definelevelname 0 5 Slaughtr.map 03:10 01:03 SLAUGHTERHOUSE
definelevelname 0 6 Minigolf.map 06:50 04:49 FUN PARK
definelevelname 1 0 Gas.map 06:15 04:12 GASSIN' UP
definelevelname 1 1 Ranch.map 05:00 03:01 HOUSE OF ILL REPUTE
definelevelname 1 2 Coffin.map 04:45 02:32 MYSTERY DINO CAVE
definelevelname 1 3 Campy.map 07:50 05:53 CAMPY CRYSTAL LAKE
definelevelname 1 4 Bigfoot.map 04:00 02:02 BIGFOOT CONVENTION
definelevelname 1 5 Hoovrdam.map 10:00 08:13 HOOVER DAM
definelevelname 1 6 Alien.map 07:10 04:55 ODDITY MUSEUM
If you don't do that it won't be able to load the levels as it will be looking for a subfolder Route66 in the .grp file. It does not exist !!! Game will go back to main window
Now use the GFS31 editor : 
https://dukeworld.co...s-grp/gfs31.zip
Insert the 30 files from temprt66 and save the grp as RT66.grp
Put it in rednukem folder as the main game grp : REDNECK.GRP which is needed to play the addon
You could also put Redneck Rides Again and rename it to REDNECKR.RTS
Final part : 
Launch rednukem
Close it.
Open with notepad the file : redneck.grpinfo in the rednukem folder
You should see listed all the .grp file available in the folder
Note the first (size) and third (crc) value of the RT66.grp line
The third value (crc) of your REDNECK.grp file
Here's mine as an example : 
"RT66.grp" 18212318 1721891171 -1860708818
"REDNECKRA.GRP" 191798609 1227896520 -1786767162
"REDNECK.GRP" 141174222 1227896404 433699961
Now open Notepad and paste the Following : 
grpinfo
{
        name       "Suckin' Grits on Route 66"
        scriptname "game66.con"
        size       18212318
        crc        -1860708818
        flags      GAMEFLAG_RR
        dependency 433699961
}
Change the values in size, crc, dependency with yours
Save the file in the rednukem directory as redneck.grpinfo (remove the .txt extension)
I tested it on both Windows and linux (copy over rednukem directory, grpfiles.cache, redneck.grpinfo and the grp files)
The only thing that did not work was the cross save which was the case with the first game
You can finally launch Rednukem and enjoy this fine addon (what a pain in the a** to make it work) and thanks to jonhunt, Man 'o War and Moggimus of duke4.net forums