For a private group, you just create a mashup and add the problems there, to create the problems use Polygon, after creating the problem grant access to codeforces, copy its link and add it to the mashup.

For a public gym, you need to have max rate of at least 1900 and exceed some number of contest participation, (forgot the exact number, I think it was about 20).

You can check this tutorial for Polygon

https://quangloc99.github.io/posts/polygon-codeforces-tutorial/