Anyone born in the 1930s, as Gaikowski was, would have quickly become familiar with secret decoder rings. Such decoder rings create substitution ciphers. And there you go. No special training necessary.
I suppose that most people learn the concept of substitution one way or another before say reaching adulthood. Gaikowski saying "Until a code is broken you have never any way of knowing whether it’s a real code or whether it’s just a fake code." is very true. With the 408 and 340 I would at least like to think that Zodiac at some point in his life had a healthy interest in cryptography. The 408 cipher and its delivery was a masterpiece in every aspect and it set up the whole Zodiac thing just perfectly.
Was cryptography a fad in the 50/60s? Zodiac researchers, logically, tend to associate it with Zodiac’s possible military/naval background.
Yes, you can find a whole assortment of crypto toys from that era on ebay. Here’s one such result (if the link doesn’t work just do a search on ebay for dick tracy code):
