The stub() function

Create a tibble that has a row column (values from 1 to 6), a group column, and a vals column (containing the same values as in row).

tbl <-
  dplyr::tibble(
    row = 1:6,
    group = c(rep("Group A", 3), rep("Group B", 3)),
    vals = 1:6
  )

Create a gt table with a two-column stub (incorporating the row and group columns in that). Format the row labels of the stub with fmt_roman() to obtain Roman numerals in the stub; we’re selecting the stub column here with the stub() select helper.

tbl |>
  gt(rowname_col = "row", groupname_col = "group") |>
  fmt_roman(columns = stub()) |>
  tab_options(row_group.as_column = TRUE)

		vals
Group A	I	1
	II	2
	III	3
Group B	IV	4
	V	5
	VI	6