Add margin row totals in dplyr chain

One option is with do

mtcars %>%
   count(cyl, gear) %>%
   ungroup() %>% 
   mutate(cyl=as.character(cyl)) %>% 
   do(bind_rows(., data.frame(cyl="Total", count(mtcars, gear)))) 
   #or replace the last 'do' step with 
   #bind_rows(cbind(cyl='Total', count(mtcars, gear))) #from  @JonnyPolonsky's comments

#      cyl  gear     n
#   <chr> <dbl> <int>
#1      4     3     1
#2      4     4     8
#3      4     5     2
#4      6     3     2
#5      6     4     4
#6      6     5     1
#7      8     3    12
#8      8     5     2
#9  Total     3    15
#10 Total     4    12
#11 Total     5     5

With adorn_totals() from the janitor package:

library(janitor)
mtcars %>%
  tabyl(cyl, gear) %>%
  adorn_totals("row") 

   cyl  3  4 5
     4  1  8 2
     6  2  4 1
     8 12  0 2
 Total 15 12 5

To get from there to the "long" form in your post, add tidyr::gather() to the pipeline:

mtcars %>%
  tabyl(cyl, gear) %>%
  adorn_totals("row") %>%
  tidyr::gather(gear, n, 2:ncol(.), convert = TRUE)

     cyl gear  n
1      4    3  1
2      6    3  2
3      8    3 12
4  Total    3 15
5      4    4  8
6      6    4  4
7      8    4  0
8  Total    4 12
9      4    5  2
10     6    5  1
11     8    5  2
12 Total    5  5

Self-promotion alert, I authored this package - adding this answer b/c it's a genuinely efficient solution here.