in R, how to calculate mean of all column, by group?

I need to get the mean of all columns of a large data set using R, grouped by 2 variables.

Lets try it with mtcars:

library(dplyr)
g_mtcars <- group_by(mtcars, cyl, gear)
summarise(g_mtcars, mean (hp))

# Source: local data frame [8 x 3]
# Groups: cyl [?]
# 
#     cyl  gear `mean(hp)`
#   <dbl> <dbl>      <dbl>
# 1     4     3    97.0000
# 2     4     4    76.0000
# 3     4     5   102.0000
# 4     6     3   107.5000
# 5     6     4   116.5000
# 6     6     5   175.0000
# 7     8     3   194.1667
# 8     8     5   299.5000

It works for "hp", but I need to get the mean for every other columns of mtcars (except "cyl" and "gear" that make a group). The data set is large, with several columns. Typing it by hand, like this: summarise(g_mtcars, mean (hp), mean(drat), mean (wt),...) it is not pratical.