Yes, the original sentence is grammatically correct. The phrase "a year in the works" means that the development of Gumroad Memberships took a year to complete. "Works" in this context refers to the process of creating or developing something. In the second part of the sentence, "creators" use "Gumroad Memberships" to earn over $1,500,000 per month. This means that the creators are using the feature of Gumroad called "Memberships" to earn money through their content.

The sentence is not well written and is quite unclear. I think it means: In November 2020 we started to ship Gumroad Memberships, which had been a year in the works. (Or - which we had been developing over the previous year) They are now being used by hundreds of creators, who earn over $1,500,000 per month in total.

My interpretation is that the company was working on developing the product for a year. After the year was over, they were finished with development and started selling it to people.