something like "collapse rows" for gt

collapse_rows.R

collapse_rows <- function(df_g, col, lookleft = TRUE){
  col_num <- grep(deparse(substitute(col)), colnames(df_g$`_data`))
  
  collapse_style <- css(visibility = "hidden",
                        border_top = "0px")
  
  test_rows <- function(x) ifelse(is.na(x == lag(x)), FALSE, x == lag(x))
  
  if(col_num > 1 & lookleft) {
    col_left <- as.name(colnames(df_g$`_data`)[col_num - 1])
    df_g |> 
      tab_style(
        style = collapse_style, 
        locations = cells_body(columns = {{ col }}, 
                               rows = test_rows({{ col }}) & test_rows({{ col_left }})))
  } else {
    df_g |> 
      tab_style(
        style = collapse_style, 
        locations = cells_body(columns = {{ col }}, 
                               rows = test_rows({{ col }})))
  }
}

Example of use:

penguins |>  
    summarize(avg_weight = mean(body_mass_g),
              .by = c(species, island, sex)) |> 
    mutate(across(1:3, as.character)) |> 
    drop_na() |> 
    gt() |> 
    collapse_rows(island) |> 
    collapse_rows(species) 

Results in this: