In Ora Maritima, paragraph 21, I find, “…sī vōbīs grātum erit…” and “Mihi quoque pergrātum erit…”.
It seems that grātum and pergrātum are accusative, masculine (mostly because I can’t see that they are anything else), but can an adjective (on its own, without a noun) be the object of a verb? And why do they have to be masculine? If I were creating this sentence (instead of translating it) why would I choose the masculine? Or anything else?
Actually I think they are nominative and neuter and complement the impersonal use of the verb “to be” - “if it is (literally, will be) pleasing to you” and “it will be also very pleasing to me”.
I haven’t seen the full text, but gratum and pergratum are neuter nominative. This is an impersonal construction. "if it will be pleasing to you . . . " English requires a subject pronoun (“it”), but in Latin there’s no need for an explicit subject in this type of construction.
The full sentences, with the relevant phrases underlined, are:
Crās, sī vōbīs grātum erit, ad locum ubi proelium erat Britannōrum cum Rōmānīs ambulābimus. (Tomorrow, if it is pleasing to you, we will walk to the place where the Britons’ battle with the Romans was.)
and
Mihi quoque pergrātum erit, sī nōbīs sepulchra Britannōrum er Rōmānōrum monstrābis. (And it would also be very pleasing to me, if you will show us the sepulchra of the Britons and Romans.)
So, what I think you are saying is that “it” is the subject of each phrase, and that the adjectives (grātum and pergrātum) modifying “it” are neuter because “it” is neuter. I can live with that - and if it’s right, all the better!
Another way of looking at this is to note that the “subject” of such impersonal expressions is always considered neuter. The reason for this is that the complementary clause is the actual subject of the verb. So in your first example, what will be pleasing? It’s walking to the place where the battle happened. In the second instance, it’s the si clause that is the subject of the verb. In our English, “it” is an expletive, a filler word we supply that makes sense out of it for us, since we don’t normally make clauses the subject of a verb.