Wow - she likes some intense shows!
one thing I would look out for is pins/badges from each show - they are really popular to collect in the country I live in (Not the UK) perhaps you can find them on Amazon/Temu etc.
Personally, I would be searching for:
Gachiakuta - harder to find things because its popularity is recent. the gloves in the show are a central part, a keychain with them on would be a cool gift.
JJK - is massive, hugely popular, loads of big brands have things from them. Clothing, soft toys etc. Uniqlo the big japanese clothing company did a collab with them a while back, maybe you can still get it online.
Tokyo Ghoul for me just makes me think of coffee culture. Does she like coffee? If she hasn't tried it because she is too young, then maybe not, but a tokyo Ghoul mug and a little coffee brewing set would be a really thoughtful gift for a fan.
For Chainsaw man - has she seen the film? If not, get that - its amazing for any anime fan who loves these types of shows. Lots of soft cuddly toys of the "dog" from the show as well (pochita) The age rating on the film is probably above her age, but it is no worse than the show, or JJK, so if you are comfortable with her watching those, it isn't any difference.
Also - in terms of other Manga that you could buy for her to try. if she hasn't already watched/read them, I would say that these would hit a similar feel to the shows you mentioned:
Kaiju no.8
Sentenced to be a Hero
Code Geass
Demon Slayer
they all have the same darker, grittier, feel that fits with the ones you mentioned. However, I would also suggest that she try some lighter anime as well - together, all of these types of shows can get pretty intense! But again, if that is normal and ok for her, then that is fine.