There's no "ready-made" red-yellow-green gradient in the preset effects, but
if you only need to use that gradient in simple shapes such as rectangles or
circles, here's a "workaround"...
Assume we want the gradient in a box, so that red is to the left, the middle
is yellow, and towards the right-hand end of the box it's "turning greenish".
1. Start out with a box, and use the two-color gradient option to make a
red-to-green gradient.
2. The middle of this will have some brownish color. Put a new box on top of
the first, and make it cover the middle third or so, so that it more or less
covers the brownish zone in the middle.
3. Make the innards of the new box a vertical gradient like the first one
has, only this one is to be the kind with one color out at the left and right
edge, and the other color near the center. The color near the middle should
be yellow and 0% transparent, wile the left and right edges go towards the
same yellow, only 100% transparent.
4. Tweak the position of the left and right edge of the second box, until
you get the right "bands" of red, yellow, and green.