the crop growth has thresholds for lighting, not enough = wont grow, more than a certain amount = normal growth, and inbetween the growth time scales.
for gleamlight, the values are set up currently so that about 3m away from a white gleam lantern you get full growth still, but the poles are so dark that even right next to the crop itd still have slow growth (but it “would” grow.)
the hud details on crops in next release will show eta on crop growth so can evaluate the lighting conditions being good or not.