why use Retrofit when we have OkHttp

with OkHttp we can make HTTP request then get response from server... then with Gson lib convert response to object we need

Note that in your code snippet, you skipped two notable steps: generating the URL and actually parsing the JSON using Gson.

So my question is what is exactly Retrofit for?

It is for generating the URL (using type-aware generated code tied to your specific REST API) and actually parsing the JSON using Gson. In other words, it does what you skipped in your code snippet.

Also, for certain types of REST operations (e.g., POST), it helps a bit in assembling what to submit (e.g., generating the encoded form).

By definition, you do not need to use Retrofit. Retrofit is computer code, written by computer programmers. Somebody else could write code to do what Retrofit does.

why Retrofit use OkHttp

Retrofit needs to perform HTTP operations. It uses OkHttp where available, for all that OkHttp provides: HTTP/2 and SPDY support, pluggable interceptors, etc.

You should use retrofit if you are trying to map your server API inside your application (type-safing). Retrofit is just an API adapter wrapped over okHTTP.

If you want to type safe and modularise the interaction code with your API, use retrofit. Apart from that, the underlying performance, request defaults, etc of okHTTP and Retrofit are the same.

Also I would recommend listening to this podcast from Jesse Wilson (developer of major android HTTP clients), where he talks in-depth of the history of development of Apache HTTP client, HTTPURLConnection, okHTTP and Retrofit.