Personally, I prefer Suze to Salers and Avèze but all make fine drinks. There are indeed more distilleries making gentian in France, Switzerland and even Germany, apparently. An interesting newcomer is the Amer Gentiane from the Distillerie de Grandmont. A really stunning product.
Gentian are another of those traditional aperitifs that modern marketing departments try to sell to younger generations mixing it with tonic. Although I love the White Negroni and its variations, my preferred serves are of the traditional French kind: with Crème de Cassis or, above all, lemon syrup (I normally do a oleo-saccharum), both topped with soda.
If French is no issue, there’s an interesting book dedicated to gentian: La Gentiane - L’aventure de la free jaune. The part on spirits goes beyond gentian liqueurs, as it include gentian-forward products such as Picon, Bonal and Secrestat.