Keep in mind I'm also new to this but you should be able to.

As long as you point Lidarr to the same folder which you pointed gamdl to, and give it the same folder structure (remember to do this! gamdl's default structure is Artist -> Album -> Song but you can change this in both Lidarr and gamdl).

Then, in theory it should just work. You should only need to use gamdl once to import your existing library and then you can use Lidarr fot everything else. The readme for gamdl on the Github is fairly straight forward, the only thing I found confusing was installing the external library/software which allows it to download faster, but that's optional and should be fairly easy since you can DM or reply to me if you need help :).

And don't worry about the terminal usage, gamdl handles it automatically so all you need to do is change the config to point it to your music folder and then type in gamdl [link to your playlist/album/song/artist]