There are 4 possible pairs Gretchen can pick, and (6C2)−3=12 ways for her to pick socks of two other colors. There are (8C4)=70 total ways Gretchan can pick socks, and thus there is a 4⋅12/70 =24/35 probability Gretchen picks exactly 1 pair.
This number might seen large, but considering there is a 2^4/ 70 =8/35 chance of picking no pairs, and a (4C2)/70 =3/35 chance of two pairs, it adds up (to 1).
https://math.stackexchange.com/questions/2660495/probability-of-choosing-exactly-1-pair-out-of-4