The most direct way available is to label the ticks, something like
library(plotly)
fig <- plot_ly(
domain = list(x = c(0, 1), y = c(0, 1)),
value = 65,
type = "indicator",
mode = "gauge+number",
gauge = list(
axis =list(range = list(0, 100),
tickmode="array",
tickvals=c(0,20,40,60,80,100),
ticktext=c("0: Wages","20", "40","60: Operating expenses", "80: Cost of sales", "100: Insurance")),
bgcolor = "white",
steps = list(
list(range = c(0, 60), color = "green"),
list(range = c(60,80), color = "yellow"),
list(range = c(80,100), color = "red")),
font = list(size = 24, color = "white", family = "Open Sans")
)) %>%
layout(
margin = list(l=100,r=100),
paper_bgcolor = "#0D0D0D",
font = list(color = "white", family = "Open Sans"))
fig