Here are 2 little Python scripts in order to create automatically shaders with 3 texture files ( Diffuse, Reflection Color and Bump ) as well as 2 gamma correction nodes for Diffuse and Reflection Color.
Select a shader and run the script, if you haven't selected any shader the script will ask you if you want to create a new one.
be carfefull, for the moment the shader works only with Vraymtl and Mia_ shader, not with other shaders ( such as SSS, Light etc. )
By default, the script applies a gamma correction at 0.455, if you want to change this value, before executing the script, type in the Python line in the bottom of maya's UI : gamma_value = "your value" and hit enter. The script will use this value until you either change the value again or re-start maya.