GBU wasn't intended to be a prequel and, in fact, all the "Man with no name" movies have Eastwood's character with a different nickname and no other characters in common, plus several actors were in multiple films playing different roles. It's rare that an actor play different roles in a series. Not so rare that different actors play the same role, but different roles for the same actors usually doesn't work well in a series with character(s) in common across the movies.
@gopher - Casino royale, as filmed was a reboot and not the original novel at all. The original novel ends with "The bitch is dead." - a line used by Bond in the movie on the sailboat just before he sets out to find Mr. White, but the story (not just incidentals) has some serious variations from the novel. For instance *all* of venice. The novel wraps up shortly after SMERSH (not Quantum) saves (and marks Bond) from LeChiffre. Vesper can't live with herself for betraying Bond and commits suicide.
But the point isn't the story and their origins, but the movies in relation to each other. In the cinematic world, Dr. No was the first and OHMSS came after YOLT (which is why there is some issue with Bond not instantly recognizing Blofeld and vice vewrs in OHMSS).
Likewise, Peter Jackson's version of the Hobbit is a prequel to Peter Jackson's version of LOTR. By virtue of the fact that license is taken with story and character any time qa book is made into a movie (where is Glorfindel at Weathertop? Where are the Rangers at Helm's Deep?) so, too, must one take into context the order in which movies were made versus their fictional chronology to decide if they are presquel or sequel.