We did this for many years. I built a concrete cistern, poured a concrete pad and used cinder block to make walls. I topped it with a shed-type roof.
I tried to seal the tank with a Portland-based material. It began to leak in a few months. Ultimately, I had to drain the tank and install a custom-made liner. For a 2200-gal cistern the liner cost me about $350. No more leaks.
I had gutter along the house roof, plumbed the downspouts into the cistern. We had lots of water. Eventually, I discovered that harvesting rain water in this manner is illegal in my state, I had to disconnect everything.
I am in freezing territory, so I had to insulate my cistern with foam panels. In your case, you wouldn't have to, I expect. And I believe you would be better off simply purchasing a polypropylene or other tank and setting in on the ground, surround it with some sort of simple shed-like structure to protect it from UV. I could not do that here, as the tank would have to be buried to protect from freezing, and my house and yard are solid bedrock. You could bury a ready-made tank but then you would not have gravity feed.
Another thing some people do is, rather than build a tank like I did, is to get themselves one of those vaults used in septic systems, the type with no holes in the sides. You would need to have a way to get it off a truck and set into position or have someone deliver it with a crane on the truck. But it would be a ready-made tank. Rumors are these vaults are pretty inexpensive, though I have not checked that.
By the way, from experience I can tell you you'll have to screen the incoming water for detritus such as leaves and pine needles or whatever. Otherwise, the outlet will eventually plug. And, you will find yourself periodically emptying the tank so you can get in there and clean out all the organic junk.
