I have been trying to install rgdal from RStudio Version 1.0.44 on ubuntu 16.04 then I was getting the gdal-config not found error. I resolved
rgdal
For Rh6, you have just to install:
yum install proj-devel-4.8.0-4.1.x86_64.rpm
N.B: in my case, my server is not connected to internet, So I download the package from repo to my local machine and then I copied it to my server.