What should you get your friend for Galentine’s Day? Everyone from Hallmark to Walmart suddenly has an idea.

Women across the country are embracing a ladies-only spin on Valentine’s called Galentine’s Day, a celebration of female friendship marked by brunches, pink drinks—and each other. “No boys allowed,” says 35-year-old project manager Amanda Nietzel, who is hosting a Galentine’s Day brunch Feb. 9 in Brooklyn, N.Y.

The...