The cells_column_spanners() function

Use exibble to create a gt table. Add a spanner column label over three column labels with tab_spanner() and then use tab_style() and cells_column_spanners() to make the spanner label text bold.

exibble |>
  dplyr::select(-fctr, -currency, -group) |>
  gt(rowname_col = "row") |>
  tab_spanner(
    label = "dates and times",
    columns = c(date, time, datetime),
    id = "dt"
  ) |>
  tab_style(
    style = cell_text(weight = "bold"),
    locations = cells_column_spanners(spanners = "dt")
  )

	num	char	dates and times
			date	time	datetime
row_1	1.111e-01	apricot	2015-01-15	13:35	2018-01-01 02:22
row_2	2.222e+00	banana	2015-02-15	14:40	2018-02-02 14:33
row_3	3.333e+01	coconut	2015-03-15	15:45	2018-03-03 03:44
row_4	4.444e+02	durian	2015-04-15	16:50	2018-04-04 15:55
row_5	5.550e+03	NA	2015-05-15	17:55	2018-05-05 04:00
row_6	NA	fig	2015-06-15	NA	2018-06-06 16:11
row_7	7.770e+05	grapefruit	NA	19:10	2018-07-07 05:22
row_8	8.880e+06	honeydew	2015-08-15	20:20	NA