Superposition is a great free tool to test your graphics card, and it's the one we used in our guide detailing how to benchmark a GPU. 3DMark is a paid program that costs $35, but you can download the 3DMark Basic Edition for free from Steam, which gives you access to a limited number of benchmarking options. You can even compare your results with others in the community to see how your system performs relative to their machines. I also like the detailed report it generates after each run, highlighting things like your GPU's score, a monitoring chart with various metrics, etc. One of the best things about the 3DMark benchmark is that it lets you customize the settings for each run, and you can even set it to run in a loop, allowing you to push your GPU to its limits and gather additional data. We run a suite of 3DMark benchmarks to test the gaming laptops, CPUs, and GPUs that show up on our testbench here at XDA, so be sure to look for the 3DMark scores in our reviews the next time you read them. Each of these benchmarks puts your graphics card through its paces as they make it render different graphically intensive scenes with pre-defined or customizable settings. It was created and developed by UL (formerly Futuremark), and it essentially includes a collection of benchmarks like Time Spy, Port Royal, and more.
3DMark is one of the best and the most reliable GPU benchmarking tools that's been around for decades.