assembly manual sect 1 sheet 83.02 says task force door gap should be .16" plus/minus .09". that's about 3/16" to 1/4" I think. lots of guys use a wooden paint stick as a gap checker. they are usually about 1/8" thick.
a place that sells plastic sheet could come up with some scrap ABS or something that is the dimension you want for your gaps. this really helps set the gaps on assembly by having the gap checker in the gap when the door is being adjusted but also helps to check the gaps after initial bolt up of the parts. like Ogre says, gap widths sorta depend on your intended use for the truck. rough roads maybe means a bigger gap than a trailer queen. 1/8" could be a little small if you plan to pull trailers or drive on uneven areas that would cause the truck frame to flex.
you can easily build a non marring gap checker by starting with a piece of wood or plastic that is the thickness of the gap you want and then laminate a thicker piece on one end of that. it works like a go-no go gauge. say you want a 3/16 gap but will allow up to 1/4. you start with a piece of 3/16 flat plastic sheet about the same width as an office ruler and a few inches long so you can get a grip on it. slightly taper the end that will slip into the gap and this will be your 1/16 gap checker. then laminate another piece of (end tapered) 1/16 back from the end of the tool just a bit, like 1/2" back. let that run back towards the rear about 1/2" and this becomes your 1/4 inch check area. then take a third piece (tapered end) and laminate it on top of that second part back about 1/2" and this becomes your sight gauge to tell how much bigger than 1/4 your gap may be . when you want to check gaps insert the tool into the gap. if it goes in and butts against the first laminated part then the gap is 3/16 gap. If it goes in up to the second laminated part then the gap is 1/4". if it goes past the 1/4 gap area of the second step but then also goes up the taper on the third laminated part then the gap is larger than you want. it is basically a graduated stepped wedge that you could also simply make by starting with a thicker piece and removing material from the stepped areas until you get the thickness you want
or buy/build one of these already made tools
https://www.youtube.com/watch?v=bJ2JoXeL0lE
https://www.ebay.com/itm/233610099279
https://www.purpledimension.com/gap-gauges.html
https://vw.snapon.com/SpecialToolsDe...itemId=5000121
assembly manual for task force
https://www.trifive.com/d1/55-59Assy.pdf