SimonZephyr: I don't think I've played any VN games. Are they all just porn? What is an "all-ages" VN?
As coates32 already explained, not all VNs are 'just porn'. Sorry about the use of 'all-ages' - it's probably more accurate to say 'without 18+ content'. I'll fix my post.
And to answer your other question:
All VNs are on a sliding scale from "no erotic content" to "every other scene is an erotic scene". Many titles that originally had erotic content also make a version with the content stripped out too (therefore making it 'all-ages' through self-censorship). Some people are uncomfortable playing these censored versions, but to each their own.
Good starter VNs without 18+ content on GOG are:
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Higurashi When They Cry (slice of life and horror mystery. First chapter is free!)
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The House in Fata Morgana (drama mystery)
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Corpse Party series (horror. RPG-VN hybrids. Start with "Corpse Party (2021)" or the original "Corpse Party")
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VA-11 HALL-A (cyberpunk slice of life with some drama, has bartending gameplay)
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eden* (sci fi drama)
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Da Capo III R (lighthearted slice of life/romance with some mystery)
There are a lot more, including some JRPG-VN hybrids, both on GOG and off, but I'd say one of the above is a good place to start.