Thanks to @DanielD.Sjoberg for pointing me towards the documentation to solve this, link here.

Corrected code is below so folks can reference this going forward.

## Use gtsummary package to create descriptive statistics table
descriptivestable <- KUWRowPowerDF |> 
  select(all_of(allvars)) |> 
  tbl_summary(
    type = all_of(c(normalvar, nonnormalvar)) ~ "continuous",
    statistic = list(
      all_of(normalvar) ~ "{mean} (\u00B1 {sd})",
      all_of(nonnormalvar) ~ "{median} ({p25}, {p75})"
    ),
    digits = all_continuous() ~ 1,
    label = list(
      `Age` ~ "Age (years)",
      `Height_cm` ~ "Height (cm)",
      `Weight_kg` ~ "Weight (kg)",
      `On-Water_Rowing_Experience` ~ "On-Water Rowing Experience (Years)",
      `Indoor_Rowing_Experience` ~ "Indoor Rowing Experience (Years)",
      `7-Stroke_W` ~ "7-Stroke Peak Power Test (W)",
      `7-Stroke_W/kg` ~ "7-Stroke Peak Power Test (W/kg)",
      `20-Second_W` ~ "20-Second All Out Test (W)",
      `20-Second_W/kg` ~ "20-Second All Out Test (W/kg)",
      `60-Second_W` ~ "60-Second All Out Test (W)",
      `60-Second_W/kg` ~ "60-Second All Out Test (W/kg)",
      `2k_Avg_W` ~ "2,000-meter (W)",
      `2k_W/kg` ~ "2,000-meter (W/kg)",
      `2k_Avg_Speed_m/s` ~ "2,000-meter Speed (m/s)",
      `2k_Stroke_length` ~ "2,000-meter Stroke Length (m)",
      `2k_Total_Time_sec` ~ "2,000-meter Time (s)"
    )
  ) |> 
   add_stat(fns = stat_fns) |>                
   modify_header(
    stat_0 ~ "**Mean ± SD**",
    add_stat_1 ~ "**Shapiro–Wilk p**",  
    label ~ "**Variable**"                  
  ) |>
  bold_labels() |>
  remove_footnote_header(columns = all_stat_cols()) |>
  modify_footnote_body(
    footnote = "Denotes non-normally distributed variable, reported as Median + IQR (p25, p75)",
    columns = label,
    rows = variable %in% c("Age", 
                           "On-Water_Rowing_Experience", 
                           "Indoor_Rowing_Experience", 
                           "20-Second_W", 
                           "2k_Avg_Speed_m/s") 
    & row_type == "label"
  ) |> 
  modify_table_body(~ .x |> 
      arrange(factor(variable, levels = allvars))
  ) |> 
  as_gt() |> 
  gt::tab_header(title = md("**Table 1 | Participant Characteristics**"),
                 subtitle = md("**N = 40; p ≤ 0.05**")
  )

descriptivestable