But there's no reasonable motive for the Zodiac Killer to do so. If he wanted to frame Gary Poaste, he would plant evidence that was more obvious. If he wanted to toy with the detectives who were trying to discover his identity though--which is well within the pattern of behavior observed from the Zodiac Killer--this is exactly the sort of thing he would do.
Such an important angle to consider this from. Until there’s a DNA match how do we know the zodiac killer just didn’t use the name of some guy he knew? Especially a seemingly uncommon name (maybe it was common at the time)
If he knew one of their suspects names and decided to fuck with them and throw them off the trail, yes, he would use someone else's name. Your leap to judgement of "it must be Gary" is just as much of a leap as "it might not be" - both rely on "knowing" the Zodiac's motives, which no one does, because he's not been identified.
No but it requires you to know apeace. Could be random letters with usernames - ok. But it's a different story with full names in the 1960s. Could have still just browsed the telephone book, but together with other hints it adds weight to the argument.