While the average user here is certainly more involved in his garage/shop than the population in general, even here I would think only a minority would want/need a gantry crane.
I happen to have one in my home garage and find it a PITA because it takes up so much room. It's a Harbor Freight model because it was A-readily available and B-cheap. I also have a cherry picker/engine crane (stored in a shed instead of the garage), but in this case I needed greater height capacity to set engines in/out of boats. I've also found it handy for lifting small boats off/onto trailers.
I certainly don't have enough use for it to make a substantially larger investment than the cost of the HF unit, even though I could easily understand the benefit of lighter weight or superior construction. Like (I assume) most here, I have one because I occasionally need it, not because I use it regularly.
In addition to competing with HF on price, you would have to compete with them on availability. Their large store network means most of us can simply go get one with a short drive. I would assume you would have to ship your product unless you distribute through a large national chain, which would of course add another layer of expense.
Your last big competitor, at least here on GJ, would be the buyer themselves. I have seen a few threads of people building their own. For home/occasional use certainly many of the members here who would have need of such a tool would also have the ability to make it themselves. Obviously this would change in a professional setting where an engineered product might be required for OSHA or other safety regulations, or where the time cost of building in-house would have to factored into the decision vs. the DIY'er who can write off his time vs. savings.
One market you might consider, though it won't be huge, is yacht maintenance. This is my field, and I have a small aluminum gantry that we can break down and set up inside a boat's salon for lifting engines or transmissions. We had it made by a local shop as a one-off, but it's proven invaluable for it's selected purpose. The small size, adjustable width, and light weight are critical for this application. I have no idea if somebody is pursuing this particular market or if it's even large enough to pursue, but it's an application where your product could be very useful.
