A popular color-wave for this watch, and I completely agree. I needed one myself, as it's the perfect summer watch!
It took me some time to get this done, it's not as popular of a watch as others, so its details aren't as easy to find.
So the thing is, the watch face isn't square. Due to the hard-to-notice asymmetry in the black/gray border the digital face is closer to 20mm length and 19mm height. I kept the dimensions of the print at 25mmx25mm so it would properly fit in place behind the built-in black/gray border (although I probably could've done slightly smaller, probably 24x24, but I figured easier to cut off ~1mm than to be too short)
I used Inkscape, since it's free
I had five nodes total for the gradient. One node at the top, one node offset at 0.23, the middle one at 0.50, then 0.77, and the last at the bottom. The top two nodes are the same color, and the bottom two nodes are the same color. This helps to make sure the richest of your colors don't get blocked by the built in border.
Once I had my colors locked in, the hard part was getting the lightness just right. I ended up with the lightness around 65% for the middle color, and at 44% for the other nodes. I tried a 35% lightness and that also came out well, just a little too dark. But anything between would probably be great, depending on preference or final color choice.
I was lucky enough to find a local place willing to print a few samples of each color/darkness for only $5 per pdf. I went with a UV print on a clear cover film. 100% opacity. This would work with LaserJet, one of my test runs had a LaserJet sample. But the fluidity and opacity just looks better with UV.
I didn't use any double sided tape. Between the perfect fit and it being UV print... I just don't think it's necessary in this build.
Next up: I think this watch would look great with a more "wintery" night sky gradient.