Gentiana saponaria, Soapwort Gentian
Smooth stem; petals that are barely longer than the "plaits" between them.

Gentiana catesbaei, Elliott's Gentian
Fine short hairs on the stem; petals that are 3-5mm longer than the plaits between them.

These species are no longer found in Delaware.

Gentiana andrewsii var. andrewsii, Closed-bottle Gentian
Hasn't been found in Delaware for several years, but can be found in nearby Pennsylvania.

Gentiana villosa, Striped Gentian
No longer found in Delaware.

Gentiana autumnalis, Pine Barren Gentian
No longer found in Delaware, and globally rare.

Sources: The Flora of Delaware, Herbaceous Plants of Maryland.