You're right, this là with accent grave has nothing to do with gender. When looking at accents/diacritics, it's worth keeping in mind that they usually either currently or historically denote a different pronunciation from the vowel without the accent, so etymologically have nothing to do with a similar accent-less spelling.
Depending on context, "cet instant est magique" could mean the same or would not specify that it is about the present instant.
Whereas "cet instant-ci" would rather mean that it is the instant that we are living now. But many French use "-là" when "-ci" would be more appropriate, therefore, context would tell.
I am not sure for English but I can tell you what it means in French:
"Cet instant-là est magique" is about a repetitive instant, something that happened in the past, not something that is happening right now at the time we speak.
J'aime quand les enfants ouvrent leurs cadeaux. Cet instant-là est magique. (I like it when kids open their presents. That very moment is magic.)
This is the best answer to the question of "-là" in this context that I have heard. The inclination to put "here" or "there" in English is of no use at all (it just leads to awkward English sentences) but this explanation hits the nail on the head! I have often said that we can't translate it if we don't know what it means. You have given it meaning. Thank you!
I tried to put: this moment here is magical, I know it's not usual english, but isn't there also this practice of putting here after this +noun to emphasize, (i was marked wrong) i.e. this picture here, or this apple here or even: that apple there. or is it alien to you all?