CUDA will use whatever your default host compiler is.
If you find that CUDA is using gcc 4.8.5 when you installed gcc 5.3, it is because you did not make gcc 5.3 the default compiler.
The default compiler is the one that will be invoked if you type:
gcc --version
or
g++ --version
at the terminal command line.
If you installed both, and then want CUDA to use the newer one after it was installed using the older one, you can use the -ccbin command line parameter to nvcc, to tell it to use something other than the default compiler.