Is the age play (gilf) aspect avoidable? - it seems like a pretty large harem most of whom aren't in that category.
Nana at least is almost unquestionably an unavoidable LI. Also, what you're asking about isn't age-play, she's just an actual grandma. Age-play is when people pretend to be a different age than they actually are during sex.
Most LIs are avoidable, and within each LI's scenes, most kinks are avoidable. Once you've made a choice about seeing a particular type of kink or not, it will either be included or not later on according to that choice.
Only a few LIs are unavoidable, and within the scenes for those LIs, you can still avoid some kinks (though not all; impregnation and pregnancy is not an optional kink for certain LIs, though the one (so far) actual pregnant-sex LI is avoidable).
Fundamentally, though, the game has ~32-36 LIs (depending which side girls end up in the final harem or not) so far. There are a LOT of them, and only a few are grandmas. I posted
a breakdown here that you can look at, but here's the summary:
Of 46 fuckable characters in the game: 6 are grandmas, with another 14 who are old enough to be grandmas but aren't (and over half of those are young enough that it's technical-only; if they had grandkids, they'd be infants or toddlers). Of the other 26, 7 are teenagers (18-19 only), and 19 are just non-teen adults too young to be grandmas (these range from 20yo to 35yo).