since the current mode buck converter has slope compensation block,
if we know current limit value from datasheet , but we don't know the slope compensation gain, how to calculate the current Amp's gain ?
Generally, the current limit is tested when it works in the low duty.And in low duty, the slope compensation is so small that you can ignore .
I don't think you can acquire the current amp's gain from datasheet. Maybe what you interest in is the transconductance of the current sense.
The simple way is to have resistor series connected to the inductor, or power MOSFET. Then you can detect the voltage drop on this resistor and convert this voltage drop into current with V-I converter.