This style for Google Calendar is similar to http://freestyler.ws/style/29516/gcal-gadget-header-gradient-color but with some different defaults:

>> centers gadget title text only on the left-hand side
>> text shadow is enabled (so white title text should be readable with just about any background color you pick)
>> uses HSL instead of RGB notation (so it should be easier to customize the color to your preference--see comments in the code)

You could also use a background color in addition to the gradient for more variety. This image shows a few colors which have been used along with the charcoal gradient (red, orange, yellow, lime, cyan, blue, purple, magenta):
http://a.imageshack.us/img827/9999/gcalgradientcharcoalroy.png
(use http://freestyler.ws/style/29648/imageshack-img-centered-w-dark-bg for a "better" view)

Color chart/samples: http://www.w3.org/TR/css3-color/#hsl-examples

Or, you can use the screen capture below to pick a color and its HSL values from the chart. For example, if you want the reddest red you can get, then the values would be '0' (for a red hue), '100' (for saturation), and '50' (for luminance [aka lightness]). So, all together, red would be: hsl(0,100%,50%). (The "funny" thing about w3's table, though, is that it's rendered using RGB [hex] notation... weird. :D)