That said, Google Scholar has Montana case law online; this link for search = easement by prescription
This link for search = frivolous claim
The Gianforte complaint appears to allege some original defect of easement grant in that grantor persons named in the complaint are alleged to have lacked full title to be able to convey an easement, with that being suggested in the complaint as having fatal consequences for continuing pubic ingress/egress. See: This site, posting by link, this pdf of the court filing.
From the first Google Scholar search; Public Lands Access v. Boone & Crockett, 856 P. 2d 525, 259 Mont. 279 (Mont. 1993) teaches as black-letter law:
To establish an easement by prescription, the party claiming an easement "must show open, notorious, exclusive, adverse, continuous and uninterrupted use of the easement claimed for the full statutory period. The statutory period is five years." Keebler v. Harding (1991), 247 Mont. 518, 521, 807 P.2d 1354, 1356. (Citation omitted.) See also; Downing v. Grover (1989), 237 Mont. 172, 175, 772 P.2d 850, 852. The burden is on the party seeking to establish the prescriptive easement. Downing, 772 P.2d at 852. "All elements must be proved in a case such as this because `one who has legal title should not be forced to give up what is rightfully his without the opportunity to know that his title is in jeopardy and that he can fight for it'" Downing, 772 P.2d at 852.
"To be adverse, the use of the alleged easement must be exercised under a claim of right and not as a mere privilege or license revocable at the pleasure of the owner of the land; such claim must be known to, and acquiesced in by, the owner of the land." Keebler, 807 P.2d at 1356-1357. "If the owner shows permissive use, no easement can be acquired since the theory of prescriptive easement is based on adverse use." Rathbun v. Robson (1983), 203 Mont. 319, 322, 661 P.2d 850, 852. (Citation omitted.)
Landowners in their McMansion had to know people were coming and going over the route in question for as long as they occupied the McMansion. Use would have been open and notorious to a grade school student; and claim of right seems admitted in the complaint - a deed of record, claimed to be defective, but sufficient as a claim of right. Ingress/egress over the route had to exist over time, well over the five year prescriptive period, even if legal title had a defect.
Neighborly accommodation and extinguishment by actions evidencing a "distinct and positive assertion of a hostile right," if present, were not pleaded in the complaint. Only a defect in legal title was alleged. If the Gianforte spouses ever put a chain or other "keep out" barrier across the access route, such possibility is absent from the pleading.
A court would have to decide whether a frivolous action was involved; but the action was dismissed by the court on its own motion for lack of any diligent prosecution of the claim.
In any event, whether a lame and unwinnable claim was or was not at issue, for a politician to pull such a boneheaded bunch of crap against public access to public facilities staggers the imagination.
That's not a legal opinion. It's a plain common sense conclusion based on a belief of there being a functioning sensible psychology to a voting public. That the quiet title stunt; valid at law or not; was a fatal error for one wanting election by a majority vote of a voting public.
Apart from practicalities and legalities of the assertion of an attempt to close off a public recreational access route by one contemplating seeking office; there may or may not be a dimension at play of The Elihu Root Quotation.
Readers may form independent opinions of these subjective "judgment" questions; and one parting thought is helpful. The Enhancing Montana's Wildlife & Habitat post, already linked, seems the most thorough and helpful analysis of the factual detail of the Gianforte route killing effort; and voter judgment shall be the determinant of the wisdom of that anti-public-interest effort ever having seen the light of day.
UPDATE: Credit for reaching the EMWH item just mentioned, is due. This fork-in-him item, included a footer UPDATE link, with the update in turn linking to EMWH.