Create a new image with a transparent background, 400px width and 125px height.
Switch to the Rectangle Select tool (R) and select the whole image. Check the box “Rounded corners” in tool options and set the value to 20. This is what you should be looking at now:
Now, pick a dark FG color, I chose #000060 and a bright version of it as the BG color, like #0000fe. Make sure there isn’t a huge difference between them tho. Pick the gradient tool and select the gradient “FG to BG (RGB)” in your tool options dialog. Click and drag a gradient from top to bottom:
Create a new layer called “white”. Go to “Select -> Shrink”, adjust the value to 5 and launch the action. This is what you should have now:
Change FG color to white. Pick the Gradient tool (L) and select the “FG to Transparent” gradient. Click and drag a gradient from top of the selection to the halfway of the image while working on the “white” layer. Then set the layer opacity to 70 %.
Now create a new layer, click and drag another gradient from the bottom of the selection towards top, around 16 pixels in length. You should see how long the gradient is at the bottom of the window.
De-select everything (CTRL + SHIFT + A) and apply a gaussian blur for the new layer at “Filters -> Blur -> Gaussian blur”, use the default values (5). Set the layer opacity to 70 %.
Add some text, in this case I added white text, I also added a drop-shadow:
As a mouseover effect, you should try flipping the background layer vertically.
Download example XCF file.