According to Wiktionary, the adjective grammatical means:
(linguistics) Acceptable as a correct sentence or clause as determined
by the rules and conventions of the grammar, or morpho-syntax of the
In the linked related question, Can “grammatical” mean “grammatically correct”?, Berrie England wrote:
To say that a sentence is grammatical is to say that it conforms to
the rules of English grammar as found in the way in which native
speakers normally use the language and... Describing any construction as incorrect is unhelpful and inadequate. That is why, in most cases, it it makes more sense simply to say whether or not a construction is grammatical.
I have seen some occasions where there are two different explanations about a grammatical issue. For example, the linked question “The earthquake, along with its subsequent aftershocks, HAS/HAVE …” asks whether it is grammatical to use have or has after "The earthquake, along with its subsequent aftershocks..."
There are two distinctly different answers posted by two users, one says we have to use has as the earthquake is the subject and along with... is a prepositional phrase, and the other says has is right, but we could consider using have as along with... has a potential to be considered as a conjunction.
Which is grammatically correct? Both of them are grammatical as long as you could quote the right reference in a grammar book.
Should we use are or is after dummy there when there are plural words following the verb? Should we use are or is after a collective noun such as family, team, etc. Can we use an indefinite article before a mass noun? Should all the English adjectives be placed before a noun? How about something special?
There are countless number of grammatical questions that could be answered in more than two ways. And some say A is grammatically correct, but B is broadly used colloquially.
What does colloquially exactly mean, then? Does it mean it is not grammatical?
I would have called you if you would have let me know it was that
Is the above sentence grammatical? Related question, “If I would have lost you” vs “If I had lost you”.
The answer is no. But it is used colloquially by some people especially in the U.S.
If A writes a grammar book that says we can use would have + PP after the conjunction if, the above sentence would be grammatical in accordance with the grammar book written by A, but it would be ungrammatical according to B, C, D, etc.
But we can't always say which book or grammar you are referring to when you say some sentences or clauses are grammatical, then, the word is as ambiguous as it gets and should be avoided unless you are sure about which grammar book you are referring to.
I think grammatical is often times synonymous with "it makes sense to my native ears" and it could be used when you talk about uncontroversial rules that are so obvious that you don't have to quote any grammar book. But saying it is grammatical should be avoided when you are not sure about what grammar rules you are referring to.