For monetization - maybe start with ads and add a https://www.buymeacoffee.com/ link or similar? I'm not convinced subscriptions would work, since I imagine that a lot of people need a gradient once and that's it, but you can always ask people.
In terms of feature suggestions:
- I like the edit colors feature - but can you make it global? i.e. I have my colors in mind, and want to see with various gradients.
- Can you convert this to code for non-CSS apps? For example, Flutter has a LinearGradient class that it seems like most of these should translate to in a straightforward way.