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Only five words put G second and R third, and just two letters ever follow that R.
E takes it in three of the five and I in two. There is no third option, which makes this one of the most constrained fourth boxes anywhere on the site.
AGREE is ordinary vocabulary and EGRET is a familiar white heron. OGRES is recognisable too, which leaves only AGRIA and AGRIN as genuinely obscure.
Five entries, a two way fourth box and three familiar words make a friendly shape. Most patterns this small are filled with dialect and technical terms instead.
If a puzzle confirms G at two and R at three, work the fourth letter first. E or I splits the group and leaves you very little left to resolve.
AGREE repeats the E, which costs part of the guess. EGRET spreads its letters better and is usually the stronger opener.
A skin eruption, an old medical term. It is the least likely answer on this page.
Grinning, in an archaic adverbial form built the same way as AGLOW and AFIRE.
GR is a consonant cluster, so it needs a vowel immediately behind it. E and I are the only two that produce real words at this length.
It makes AGREE easy to spot but weak as a probe. Repeats are rare here, one out of five, and AGREE is the one.