The bad guys in the book are
actual far-left terrorists (modeled on the German far-left group Red Brigade,) and Gruber was known as "the red" (as in socialist.)
Surprisingly... leftie Hollywood instead turned them into a bunch of greedy euro-yuppie "pretend terrorists" battling a great American working class hero who channels Americana tropes in a film released the same year the Savings and Loan Crisis really got going - Americans screwing over Americans.
All produced by one of the most repugnant human beings this side of Harvey Weinstein, Joel Silver (whose exec ranks also featured some total low-lifes, though an exec there who later became Mrs RDJ was not) and directed by a guy who did prison time for wiretapping his ex-wife - take that, Mrs McClane!
Bankrolled and released by a studio - Fox - that at the time the movie was developed was co-owned by a guy who became a fugitive for tax evasion and illegal trading with (financier of terrorism) Iran during the Iran hostage crisis - Gruber would be proud - and was later sold to Rupert Murdoch, who pioneered the very type of scumbag tabloid reporting seen in the movie in precisely that era.
Those factoids never seem to make it onto these types of "did you know?" YT videos.
**** Die Hard.