soil type depends on which gentian you buy as some need ericaceous compost whilst some species don't mind not having it. most require good drainage as they are mostly alpine type species.
when you know which ones you plan to buy look up their cultural needs using the RHS website.

Spring flowering ones like G. verna need lime. Autumn flowering ones like G. Kingfisher etc. hate lime.
Friend who grew them for showing always swore by top dressing in Summer with sheep droppings and giving them plenty of rain water.

Thank you Seaburngirl and owdboggy,didn’t realise about different soil for different plants will check out RHS site when I decide what one buy.

I've found G. acaulis is pretty easy to grow in a raised bed, and it's self-seeding there as well