Create three video layers
On the top layer you place your text media with white letters and a transparant background
On the middle track you place a media generator event with the gradient you would like to appear into the text (or any other picture or video you like to appear into the text).
On the lower track you place the background file.
Next add the MASK GENERATOR FX to the upper track with the text and set it to LUMINANCE.
Make the COMPOSITION MODE of the uppertrack to MULTIPLY (MASK)
Make the middle track a child of the upper track.
That should do it.

Theo