Calculating Gate Resistor for IRFP260N
A very good place to start is the test conditions of the part. In this case that is 1.8R-25R (say 10R). This will be a very good and safe place to start as your initial powercore design as the other numbers from teh datasheet (losses etc) will be derived from using this value.

once you have a gatedrive and a powercore designed,  a level of specific checking can be done to retune the gate resistor with respect to switching characteristics. 

what the best way to calculate Rg gate driver for Mosfet