The minimum gate triggering current is the smallest current you need to give the SCR to guarantee triggering under normal working conditions.
This is the amount of current the rest of your circuit needs to provide. If your circuitry provided less than this the operation could fail to function properly under some conditions. For example high-temperature, low temperature or with a device that is not as sensitive but still meets the datasheet specification.
This current requirement is not calculated as such. The device will be designed to meet a specific minimum gate triggering current. Every device will be tested when manufactured but only under a specific set of conditions. As a circuit designer you will need to calculate the minimum gate current your driver circuit will provide and ensure it meets the minimum requirement of the SCR.
The minimum trigger current is not very dependent upon the voltage between the anode and cathode until the breakdown voltage is reached, however it may be significantly dependent upon the device temperature with much more current being required at low temperatures.
For example see figures 3 and 5:
C106 SCR datasheet