Disputandem is most likely an error of memory which the editors - most likely not experts in Latin - missed.
The word should be disputandum - the masculine ablitive passive present participle of disputo, -are “to dispute.” It is also known as a Gerundive.
There is some confusion in your terminology here. I will try to clear that up. disputandum is actually the neuter singular nominative gerundive (sometimes called “the future passive participle”). There is not, in fact, a passive present participle in Latin (except in deponent verbs, which is another topic).
I decided it must be in the ablitive as the participle must agree in gender and gustus, -us is a masculine noun.
You are right to look for agreement between participles and nouns, but “disputandum” cannot be in the ablative. If it were agreeing with “gustibus,” the correct form would be “disputandis” (the ablative plural masculine gerundive), which does not appear. Furthermore, supposing you had “De gustibus non disputandis est” - forgetting for the moment the confusing word order - where is the subject or predicate? “Non est” would have nothing to express.
What we have here is called the passive periphrastic, a nasty name that basically refers to the combination of a form of “esse” (in this case, “est”) with the gerundive. I’m not sure if you’ve learned this as of yet in your study of Latin, but a common example is: “Karthago delenda est.” Here, “est” and “delenda” combine to form the passive periphrastic, which expresses obligation or necessity. Thus, it is translated as “Carthage must be destroyed.”
So, in “De gustibus non disputandum est,” where is the subject? Another tricky part: “non disputandum est” is impersonal: there is no explicit subject. We can translate it by supplying an “it,” as follows: “It must not be disputed.” The “de gustibus” means, as you perceived, “about tastes.” Literally, then, “it must not be disputed about tastes.” In better English, “Don’t try to argue about tastes.”
I’m sorry if this post is long-winded, but the collision of two fairly tricky Latin concepts made the sentence difficult to tease out with brevity. I hope this helps.
Regards,
David
PS - Careful with the spelling of “ablative!” The Latin is “ablativus,” which comes from “ablatus,” the past participle of “aufero,” to steal, to carry off.
PPS - I think some of my points address concerns that you edited while I was in the middle of composing. Please ignore whatever is no longer relevant.