What do these purple stems mean on my gorilla grow auto?

Can mean so many things my grownie, temp, stress, genetics, your nutrients could trigger it as part of beneficial salt stress. Red petioles aren't neccesary cause for concern, you could think of it as inflamed skin (or a sun burn)

magnesium deficiencies often cause the petioles (the little stem that connects the leaf to the stem) to turn red. When the plant starts to flower, it uses more calcium and magnesium, give it a bit of calmag. If you've given it before and you're sure it's not a magnesium deficiency, it could be due to genetics or stress (too much cold or too intense light). If you've applied any low-stress training or defoliation methods, it's possible that over time the stems of your plants will take on a red, pink, or purple hue. This is completely normal.