Actually it's quite easy for a native Persian speaker.
The German "G" is the same like the Persian گ (IPA [g]).
The German "R" depends on the region. In the South it is similar to the Persian ر (IPA [r]). But in the standard variant it is like a French r (IPA [ʁ]) or the Persian غ (IPA [ɣ]). But depending on the position in the word and the surrounding vowels and consonants, the "r" can be pronounced differently.
The best is, if you watch some of the many videos introducing German pronunciation on youtube. For example this one, where the pronunciation of "r" is explained: https://www.youtube.com/watch?v=fLNPsFOnTWI
Maybe try to think of the "R" like a very soft hissing sound (like the German "CH"). Don't roll the tongue, but rather keep it flat in your mouth. Imagine a cat hissing... xD
As for the "G", it's a soft "K". If you can say the German "K" you'll realize that there's a lot of air coming out, when you say it. Try to reduce the air to a minimum, so if you hold a lighter in front of your mouth the flame won't move a bit when you say "G".
Hope that was somewhat helpful :)