Can someone please help my gc are going light gree...

This looks not well, and I think it improbale that it is a lack of nutrients, since the plant is still small compared to the pot, so it would still thrive perfectly in this amount of ordinary potting soil from the supermarketwith nothing added. This looks like a drowning plant, or something has built up in the soil becoming toxic for the plant. Soil PH? I don't think so, Ordinary potting soil for petunias and flowering plants people want on their balcony is perfectly fine so I suspect that this plant is rather suffering from too much of something than a lack of nutrients.

Since you're not telling us in your diary that you're feeding them at all, I think the problem might be that they desperately need some food! Get them on a good NPK nutrient - if this is a photoperiod, start them of with 1/2 strength... if it's an autoflower, start with 1/4 strength... you can always increase the dose if the plant tolerates the reduced amounts... I'm not seeing any info on the pH you're maintaining and this will be critical as well.... make sure the pH is between 6 and 6,5.... Good luck!

leaves look stacked on top of each other. Raise light or reduce power. a streak in teh stem can be low sulfur. red petioles can be related to low phosphorous. if your leaves are popping out very close together, get your light in order first as that may clear up a lot of it. you need 66% of the power that you would need in a 12/12 schedule vs 18/6. hps is good at 50w per sq ft for 12/12. so you'd only need 33w per sq ft in vege. hope this stands out, if not it's mostl nutrient related and you should up the nitrogen / phosphorous and probably sulfur too if you got a red streak in stem. sometimes it can be a bit genetics too. i've given plenty of p/s to the boundary of toxicity and still had red stems before.