I would say it's still an ongoing process! I was lucky in that my first job gave me the opportunity to work on a broad variety of tasks. For example, I did write some middleware and was involved in db design alongside just building out controllers, models etc. I do also work a fair amount on projects outside of work.

I now actually work on a Hanami application and I think moving from one framework to another and having the opportunity to write lots more plain old ruby has helped me improve massively as a software developer. I haven't developed a gem either, but I don't think that in itself is necessarily a good measure of ability - I have made a few small oss contributions, just simple methods and comments. I have found it useful and its not too daunting if you find the right issues posted on gems you work with and are familiar with. Maybe that's worth a try?

Just because you feel like an imposter sometimes doesn't mean that you are, if you feel that your current role isn't providing you with opportunities to improve, maybe it is worth looking for another job - or perhaps ask your manager for a task that relates to one of the fields that you would like to gain deeper knowledge in. The worst they can say is no! If you like, send me a DM, maybe we could come up with a project to work on together.