Letters group ordering in Games Howell post hoc in R

Question

I want to get a connected letters report from a Games Howell post hoc test in R. I have achieved it using several options, for example:

library(PMCMRplus)
GH_vol <- gamesHowellTest(Tree.volume ~ Rootstock, data = Olives_GH)
summaryGroup(GH_vol)

library(userfriendlyscience)
oneway(y = Olives_GH$Tree.volume, x = Olives_GH$Rootstock, posthoc = 'games-howell', posthocLetters = TRUE)

The problem is that the connected letters report I get is not ordered by the mean values of the numeric field (Tree.volume), but by the name of the factor (Rootstock). I mean, the "a" is not assigned to the value with a higher mean, but to the value that is the first by alphabetical order of the factor.

Does anyone know how to order it by the mean?