First thing you will need is a handheld microscope to check the trichomes. If you look now you will notice most are clear, first stage, some may be cloudy, second stage and you may not see any yet that are amber, third stage. There are three microscopes that work best. First is the loupe like a jeweler, it's a joke but some people use them. Second are the handheld digital microscope they work fine but are a pain in the butt trying to focus they cost about $20 bucks on Amazon. Last is the digital handhelds, mine I believe is an Orbital, you scan it over the plant and then open up the file on your computer and zoom in at up to 100x magnification. These give you the best overall picture and are the easiest to use mine ran $50 on Amazon. Cool thing to is you can save the file and compare it the next one you take, being a first time grower this will help you tons understanding the final stages of flowering. I didn't find these till I was an experienced grower and I still learned a ton.
When to harvest has to do with what kind of high you want. GSC is a sativa dominant blend which inherently gives you the body high and is less lethargic than an indica blend. If the body high is what you are looking for harvest when roughly 10-20% of trichomes are amber. If you are looking for more of a lethargic high wait longer. You don't want to go past roughly 70-80% amber in my opinion and that's pushing it.
Ps I noticed this is your first grow. If it doesn't come out great keep it up it will come. Learn from your mistakes and capitalize on your successes. Just a piece of advice GSC is a difficult strain to perfect even for experienced growers. Good luck bro!
