Ouch.
The fundamental problem isn't so much that your GPU is underpowered. Generally speaking, an underpowered GPU simply won't work, not simply get fewer frames per second (though there are some edge cases).
The problem is much different and much worse. You're powering a modern GPU that costs $500 with a power supply I wouldn't pay $500 in money from the game of Monopoly for. Having a quality power supply is the most important part of a PC build and you're powering your expensive component with a junk-tier PSU. The manufacturer of these cheapo store-brand power supplies, Huntkey, is notorious for throwing in the cheapest, dodgiest Chinese components they can find.
I wouldn't even use the GPU with this PSU installed. This isn't a replacement to get around to when you have a chance, this is like a flashing oil light on your car's dashboard. Pull over to the side and call a tow truck.