For the sake of establishing providence, I've assumed that a mashup of the words "gnarly" and "hardtail" was where this terminology began, and it can give us a starting point to unpack the concept of "Gnartail". In any case, I take it to mean a hardtail mountain bike that is somewhat "gnarly", or perhaps stated a better way - progressive. Quite modern.
Above: Kris Viklund shredding on his custom KRUCH Berserker Gnartail
In mountain bike lingo these days progressive/modern means a bike has been constructed to be long and low slung. It means the rig is "slack" (head tube angle). It means the frame has plenty of stand over clearance. It means there is room for a big (175mm or more) dropper seat post, and room for big wheels (29" or 27.5+) coupled with aggressive tires. Most new and progressive bikes are able to accommodate fork travel numbers of 150mm or more.
And because these attributes make progressive mountain bikes very capable, a gnartail then should be built to take some serious punishment. It needs to be strong.
It should also climb well. Progressive geometry is no more apparent (and useful) than when gravity takes hold and terrain gets steep, fast and loose. But as they say - you have to get up - to get down.
A steep seat tube angle makes long seated climbs much more efficient, and perhaps more importantly - pleasant.
Above: A pair of KRUCH Gnartails ready to go, for the long, loose descent ahead.
I had built seven hardtail mountain bikes in this style, each one more progressive than the last, when I decided after some friendly peer pressure to build one for myself. The feedback from each of the riders from this first cohort of progressive frames was universal - they reported that the bikes climbed very well and descended even better. In fact one of these early adopters let me know that his suspension bike was gathering dust - his rig of choice was now his Gnartail.
As a larger rider my personal bike got thicker walled 4130 chromoly tubing than the first examples, rectangular 4130 chain stays, and was made deliberately long. Longer than my XXL suspension bike (A Starling Murmur). Since it was just for me, I could experiment a bit. I decided on a 62 degree head tube angle, and a 77 degree seat tube angle. Pretty extreme stuff.
Above: The Author's personal KRUCH Gnartail in CAD
However once I rode my own, more progressive version of the same type of hardtail - it didn't feel all that extreme. Frankly the bike just worked, and it worked really really well. It was predictable and sure-footed, and dampened vibrations very well.
Above: I call it the Foreboding Fritter. For obvious reasons.
A friend of mine had purchased a Kona ESD (another modern progressive hardtail this season, and found it to be unsure, and twitchy at low speeds. He ended of selling the bike.
My experience aboard my own rig was totally different than this but it may have something to do with the 29+ wheel size I chose to go with. The massive tires really settle the ride and offer massive amounts of traction when required.
Above: Local rider Sarah Brunswich. One of the fastest most technically proficient riders I know, on her KRUCH Gnartail.
So, where to go from here? What do future versions of this bike look like?
The tweaks I have been considering are as follows:
T47 Bottom Bracket - for more versatility and brute strength.
Konga weld yoke - for strength and for clearance for up to a 3" wide studded 29+ tire.
Paragon Machine round dropouts - modular, clean looking - and light.
Super boost (157mm) rear end - for a stronger wheel bracing angle, better chain-line, and to be future proof.
63 degree HTA and 77 degree STA with 160mm travel fork installed. Progressive but accessible.
Room for a 200mm rear brake rotor.
Straight gauge 4130 seat stays for a slightly stiffer ride.
Ideally a rider can get three solid seasons of riding out of this style of bike. Late Fall, Winter (on studded plus tires), and Spring. And pull it off the wall when the suspension bike needs service!