Should you need to target only the row group column for column-width
declarations (i.e., when row_group_as_column = TRUE is set in the initial
gt() call), the row_group() select helper can be used. This shorthand
makes it so you don't have to use the name of the column that was selected
as the row group column.
Examples
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).
Create a gt table with a two-column stub (incorporating the row and
group columns in that). We can set the widths of the two columns in the
stub with the row_group() and stub() helpers on the LHS of the
expressions passed to cols_width().
tbl |>
  gt(
    rowname_col = "row",
    groupname_col = "group",
    row_group_as_column = TRUE
  ) |>
  fmt_roman(columns = stub()) |>
  cols_width(
    row_group() ~ px(200),
    stub() ~ px(100),
    vals ~ px(50)
  )
See also
Other helper functions:
adjust_luminance(),
cell_borders(),
cell_fill(),
cell_text(),
currency(),
default_fonts(),
escape_latex(),
from_column(),
google_font(),
gt_latex_dependencies(),
html(),
latex(),
md(),
nanoplot_options(),
pct(),
px(),
random_id(),
stub(),
system_fonts(),
unit_conversion()