The cells_column_labels() function

Use sza to create a gt table. Add footnotes to the column labels with tab_footnote() and cells_column_labels() in locations.

sza |>
  dplyr::filter(
    latitude == 20 & month == "jan" &
      !is.na(sza)
  ) |>
  dplyr::select(-latitude, -month) |>
  gt() |>
  tab_footnote(
    footnote = "True solar time.",
    locations = cells_column_labels(
      columns = tst
    )
  ) |>
  tab_footnote(
    footnote = "Solar zenith angle.",
    locations = cells_column_labels(
      columns = sza
    )
  )

tst1	sza2
0700	84.9
0730	78.7
0800	72.7
0830	66.1
0900	61.5
0930	56.5
1000	52.1
1030	48.3
1100	45.5
1130	43.6
1200	43.0
1 True solar time.
2 Solar zenith angle.