Hey there,
A bit tricky, but also straight forward. I think you sent us a file that was a work in progress. The material node in the file was not the same as the screen capture, and the UVs were kinda mixed up. But I was able to recreate everything.
The remedy for a Diagonal gradient across a transparent cube with nodes takes two steps.
First - I added some math to the node to capture diagonal movement. I simply Added X and Y together and then subtracted 1.0 - this sent an appropriate Position value to the Gradient Node to create a diagonal gradient.
Well this only get's you part of the way there. If you drop this material on a default Cube - each face will have the same diagonal gradient for that face. I am guessing since this is a transparent cube, you were wanting a Diagonal gradient for the cube.
So, the second step is to go into the UV editor. Select all polys. Activate the UV Mapper. In the TOOL tab, Change the "Mapping type" to FLAT. Hit OK on "Write UV Coords." That's pretty much it. But there are some limitations. With this setup you won't be able to place the Periodic Info on all sides. With this setup there is a very specific FRONT & BACK to the cube and you will not be able to place the periodic info on the top, bottom or sides. To guarantee that the Periodic info is on one face I created two materials (one with text and one without) and applied them appropriately.
If you want the Periodic info on all sides WITH a diagonal gradient for the cube - I would have to ponder that more. Be sure to show us your final.
Cheers.