https://www.mangago.zone/read-manga/kore_wa_koi_no_hanashi/ - controversial manga, but story is really well written. ML&FL knew each other since she was 10 years old; 20 years age gap
https://www.mangago.zone/read-manga/sesame_salt_and_pudding/ - also 20 years age gap, but FL and ML are adults here, really healthy relationship. I recommend this one
Sorry I didn't notice that you wanted manhwa. Both above are manga.
But I know manhwa too. You can try:
- https://www.mangago.zone/read-manga/marriage_of_convenience_scylla_scans/
- https://www.mangago.zone/read-manga/i_am_a_child_of_this_house/
thanks (￣▽￣)
straight manhwas with age gap