Key Gearings 1 in refers to an optional of predefined constant numbers g that multiply the applicable fixing in Index 1 in order to produce the final rate (before the spread addition) used in calculating the floating cash flow amount.
If omitted, a flat value of 1 is assumed.
If n values are provided, then the first n cash flows are paired with the provided n values in successive order.
If any cash flows are left over, they will use the last value.
Look at for the exact mathematical formula.