The fmt_scientific() function

Use exibble to create a gt table. Format the num column as partially numeric and partially in scientific notation (using the num > 500 and num <= 500 expressions in the respective rows arguments).

exibble |>
  gt() |>
  fmt_number(
    columns = num,
    rows = num > 500,
    decimals = 1,
    scale_by = 1/1000,
    pattern = "{x}K"
  ) |>
  fmt_scientific(
    columns = num,
    rows = num <= 500,
    decimals = 1
  )

num	char	fctr	date	time	datetime	currency	row	group
1.1 × 10−1	apricot	one	2015-01-15	13:35	2018-01-01 02:22	49.950	row_1	grp_a
2.2	banana	two	2015-02-15	14:40	2018-02-02 14:33	17.950	row_2	grp_a
3.3 × 101	coconut	three	2015-03-15	15:45	2018-03-03 03:44	1.390	row_3	grp_a
4.4 × 102	durian	four	2015-04-15	16:50	2018-04-04 15:55	65100.000	row_4	grp_a
5.5K	NA	five	2015-05-15	17:55	2018-05-05 04:00	1325.810	row_5	grp_b
NA	fig	six	2015-06-15	NA	2018-06-06 16:11	13.255	row_6	grp_b
777.0K	grapefruit	seven	NA	19:10	2018-07-07 05:22	NA	row_7	grp_b
8,880.0K	honeydew	eight	2015-08-15	20:20	NA	0.440	row_8	grp_b