There is an easy way to do this:
1) Select gradient tool.
2) Open the colour tab, then click More >>.
3) Select your primary colour to the gradient colour, then click the secondary colour.
3) (MAKE SURE YOU ARE ON SECONDARY) Where it says Opacity - Alpha change the slide to 0.
4) Then use the gradient tool like you would normally, this should not replace the previously created one.
NOTE: The secondary colour will not matter, it just matters about the Opacity - Alpha.
Hope this helps ;D