This is not quite right... from the code, you are dividing a character by a character column. Do you see this ? The syntax is not correct for R code.
mutate(gss, PercentOfMarried = mean(marital == "MARRIED"))
may result in what you want. marital == "MARRIED" would be TRUE when the respondant answered MARRIED and the mean would be equivalent to your percentage I guess.
Hope it helps