问题是，如果数字为零，round()不会显示数字。我们通常使用sprintf。我会用gt::fmt_number()来格式化。这里我们可以指定columns和rows以及decimals：

library(dplyr)
library(tidyr)
library(gt)

a |>
  group_by(year, SampleDate) |>
  summarise(across(c(F:LF), sum),
            count = n()) |> 
  ungroup() %>%  
  mutate(across(c(year:SampleDate), as.character)) |> 
  add_row(
    summarise(a,
              year = 'Summed',
              SampleDate = '- - - - -',
              across(c(F:LF), sum),
              count = n())
  ) |>
  gt() |> 
  fmt_number(
    columns  = c(F:LF),
    rows     = year == "Summed",
    decimals = 2
  )
#> `summarise()` has grouped output by 'year'. You can override using the
#> `.groups` argument.

数据来自OP

a <- structure(list(SampleDate = structure(c(15710, 15713, 15713, 
                                               15710, 15710, 15713, 15713, 15710, 15708, 15713, 15712, 15708, 
                                               15708, 15713, 15712, 15708), class = "Date"), year = c("2012", 
                                                                                                      "2013", "2013", "2012", "2013", "2013", "2013", "2013", "2013", 
                                                                                                      "2012", "2013", "2013", "2013", "2013", "2013", "2013"), F = c(0, 
                                                                                                                                                                     1, 0, 0, 0, 1, 0, 0, 0, 22, 0, 0, 0, 65, 0, 0), W = c(0, 0, 1, 
                                                                                                                                                                                                                           0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0), S = c(0, 0, 0, 0, 1, 
                                                                                                                                                                                                                                                                         0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0), LF = c(1, 0, 0, 1, 0, 0, 0, 
                                                                                                                                                                                                                                                                                                                  1, 0, 0, 0, 0, 1, 0, 0, 1), week = c("01", "02", "02", "01", 
                                                                                                                                                                                                                                                                                                                                                       "01", "02", "02", "01", "01", "02", "02", "01", "01", "02", "02", 
                                                                                                                                                                                                                                                                                                                                                       "01"), month = c("January", "January", "January", "January", 
                                                                                                                                                                                                                                                                                                                                                                        "January", "January", "January", "January", "January", "January", 
                                                                                                                                                                                                                                                                                                                                                                        "January", "January", "January", "January", "January", "January"
                                                                                                                                                                                                                                                                                                                                                       )), row.names = c(NA, -16L), class = "data.frame")

gt具有创建汇总行的函数：grand_summary_rows()和summary_rows()。您可以使用fmt_numbers()控制显示的小数。您可以添加另一个grand_summary_rows()调用，以便为不同的列集添加第二个汇总行。如果要计算同一组列的多个汇总行，可以在fns参数中list()多个汇总行规范。在下面的示例中，我将mutate_at()更改为mutate(across(...))表达式，它取代了mutate_at()等。

upper_conf <- function(x) sum(x) + sd(x) * 1.64

a |>
  group_by(year, SampleDate) |>
  summarise(
    F = sum(F),
    W = sum(W),
    S = sum(S),
    LF = sum(LF),
    count = n()
  ) |>
  ungroup() |>
  mutate(across(year:SampleDate, as.character)) |>
  bind_rows()  |> 
  #select(-year) |> 
  gt() |> 
  # control decimals for non-summary rows here
  fmt_number(is.numeric, decimals = 2) |>
  grand_summary_rows(
    columns = F:count,
    fns = list(label = "Total", fn = "sum"),
    # control decimals for summary rows here
    fmt = ~ fmt_number(., decimals = 2) 
  ) |> 
  grand_summary_rows(
    columns = F:LF,
    fns = list(
      list(label = "Std. Error", fn = "sd"),
      list(label = "Upper Conf. Int.", fn = "upper_conf")),
    # control decimals for summary rows here
    fmt = ~ fmt_number(., decimals = 2) 
  )