Well, I wonder if we have a one-post-wonder here. Anyway, factory training is always a good thing to have on your qualifications. Being certified will help you with your insurance which for GD's is expensive. And being certified helps sales.
Also, even though GD's are mostly a one man installation, it won't hurt to have 2 in the beginning. Hire someone with very good experience and when you grow, send him out on a 2nd truck. OTOH, it takes almost a full time person to measure, estimate, sign contracts and order. Be that person and show up on most installations and you will get a good rep out of the box. That in turn will get you referral business which is the most profitable. Advertising will probably cost you $100 per sale in the beginning. Get a referral and your cost of sale goes down to 50 bucks, and so on. If you don't advertise, you have to have some other form of marketing.
Sent from someone who sold and installed windows and doors for 25 years. I know the drill. Almost got into the GD business until the actual supplier talked me out of it. He told me that GD companies were a dime a dozen (correct), very competitive (correct) and that I'd need a new kind of insurance which was expensive.
I decided to stay put with the doors and windows.