I will not be critical of your choice, but I am critical of your view on CFIs and the value that it can bring to make you a better and more professional pilot. While it is not a wonderful job by any means, you do learn more there that will help you in the 121 world than just paying to sit in the right seat. 121 operations are not hard at all. When in your flying experience have you had to make decisions? Should the first place to make real decisions be your first time in a 121 cockpit? While you are assisting in decision making as an FO you never have the final call. You do get some of that experience instructing.
Why even bother with gulfstream in the first place? How much money would it cost to get your commercial multi engine rating? My old company is hiring guys with 300 hour in the right seat of an RJ and you will get paid as an FO. There are better ways to go about getting into professional aviation that the route you are looking at.
It probably will not cost you a job, but there are a lot of ill feelings toward guys who pay to sit in the right seat of a 1900. That is a job that someone should be paid to do. Not only are you doing it for free, but you are paying someone to do it. There is something very wrong with that. Would you pay to sit in the right seat of an RJ? What about a 737?
There is much more to being a professional than time and flying. You will be at a regional for a few years and get to experience the crappy contracts and lifestyles that are a result of people just trying to get their time to make it to that next step faster. But you need to always remember that you might never make that next step.