Khmer/Cambodian letter translation?

I am planning on getting a Tattoo with my first initial alongside my 3 cousins in Khmer script. I know that Khmer is NOT a direct translation of the english alphabet but I would like to find the best comparable letters. the letters are "RRGM" I know that in khmer there are letters that sound like "mo" "ra" and "ro" which are three I definetly need, but I have not been able to find something like the "G" sound. I would appreciate the help!!