Well, you bought it. That sounds flippant but in all seriousness, that means people buy them. There is a saying that there is no problem that the right price won't cure. In other words, price what you have appropriately for the market and someone will buy it. Corollary, if it won't sell, adjust the price.
Grand Banks has good brand recognition and deserves it, so you have that going for you.
But I'd add something to that, which is, that there is also a certain "band" of value wherein the identical thing could be at the top, middle, or bottom of the band. The difference? Ad presentation. Doesn't need to be glossy or catchy, but as a buyer I will go "top of the band" for a boat that is advertised clearly, with good photos, engine data, electronics models, clear price, etc. Don't make me beg for snippets of information.
Conversely, if the boat photos are poor, the boat is full of stuff, there is no price, details are vague, and it just generally seems like I'm going to have to do a lot of work to even get a clear idea of it, then I'd be bottom of the band -- or maybe just move on to another boat, if it's one that is not super rare.
I suppose a starting point is what you paid for it, plus 50% of the cost of your improvements (total ballpark obviously). You could also look at Yachtworld for ideas; there are probably a couple dozen fiberglass GB 36's listed right now. (Your profile says 1972, which would be wood; your OP here says 1976, which would be fiberglass. I think 1973 was the transition year.)
