You’re after a rather bespoke table that requires some bespoke coding. Hopefully this helps!

The general outline here is that we can use the cards and cardx packages to perform the pairwise tests, then you need to use p.adjust() to perform the correction to the p-values.

library(gtsummary)

set.seed(123)
df <- data.frame(
  group = sample(c("A", "B", "C"), 100, replace = TRUE),
  continuous_var_1 = rnorm(100, mean = rep(c(5, 6, 7), length.out = 100)),
  continuous_var2 = rnorm(100, mean = rep(c(5, 6, 7), length.out = 100)),
  categorical_var_1 = sample(c("Yes", "No"), 100, replace = TRUE),
  categorical_var2 = sample(c("Yes", "No"), 100, replace = TRUE)
)

# function to perform pairwise fisher test
fisher_pairwise <- function(data, variable, by, ...) {
  cards::ard_pairwise(
    data = data, 
    variable = all_of(by), 
    .f = (df) cardx::ard_stats_fisher_test(data = df, by = all_of(by), variables = all_of(variable)) |> dplyr::filter(stat_name == "p.value")
  ) |> 
    purrr::imap(~dplyr::tibble(column = cardx::bt_strip(.y) %>% paste0("**", ., "**"), p.value = .x$stat[[1]])) |> 
    dplyr::bind_rows() |> 
    tidyr::pivot_wider(names_from = column, values_from = p.value)
}
fisher_pairwise(data = df, variable = "categorical_var_1", by = "group")
#> # A tibble: 1 × 3
#>   `**'A' vs. 'B'**` `**'A' vs. 'C'**` `**'B' vs. 'C'**`
#>               <dbl>             <dbl>             <dbl>
#> 1                 1                 1                 1

# function to perform pairwise kruskal-wallis test
kruskal_pairwise <- function(data, variable, by, ...) {
  cards::ard_pairwise(
    data = data, 
    variable = all_of(by), 
    .f = (df) cardx::ard_stats_kruskal_test(data = df, by = all_of(by), variables = all_of(variable)) |> dplyr::filter(stat_name == "p.value")
  ) |> 
    purrr::imap(~dplyr::tibble(column = cardx::bt_strip(.y) %>% paste0("**", ., "**"), p.value = .x$stat[[1]])) |> 
    dplyr::bind_rows() |> 
    tidyr::pivot_wider(names_from = column, values_from = p.value)
}
kruskal_pairwise(data = df, variable = "continuous_var_1", by = "group")
#> # A tibble: 1 × 3
#>   `**'A' vs. 'B'**` `**'A' vs. 'C'**` `**'B' vs. 'C'**`
#>               <dbl>             <dbl>             <dbl>
#> 1            0.0722             0.288             0.624


tbl <- df |> 
  # create the primary summary table
  tbl_summary(by = group) |> 
  # add the standard comparison across the three groups (A, B, C)
  add_p(
    test = list(all_continuous() ~ "kruskal.test",
                all_categorical() ~ "fisher.test")
  ) |> 
  # add custom stat, that is, the pairwise comparisons
  add_stat(
    fns = list(all_continuous() ~ kruskal_pairwise, 
               all_categorical() ~ fisher_pairwise)
  ) |> 
  # style the pvalues with the pvalue syling function
  modify_fmt_fun(contains("vs") ~ label_style_pvalue()) |> 
  # apply p-value correction (you may need to change `p.adjust(n)` depending on how you planned to make your correction)
  modify_table_body(
   (table_body) {
     # get all the pairwise pvalue colnames
     pvalue_colnames <- select(table_body, contains("vs")) |> names()
     # perform correction to p-value
     for (v in pvalue_colnames) {
       table_body[[v]] <- p.adjust(table_body[[v]], n = 12, method = "fdr")
     }
     table_body
   }  
  ) |> 
  modify_spanning_header(contains("vs") ~ "**Pairwise Comparisons**") |> 
  modify_footnote(
    contains("vs") ~ "Pairwise p-values corrected for multiple comparisons using the false discovery method"
  )

^{Created on 2024-12-21 with reprex v2.1.1}