gradient tool in illustrator - Graphic Design Stack Exchange
PLEASE HELP! Gradient tool won’t work
Cannot colour gradient??
How do you add gradient to entire object (different colored fills) to create a shadow? Illustrator CS5. Thank you.
Videos
Those little diamonds are sliders to choose which color should be more dominant between the two colors. (Slide it to choose.) By default it is 50% (equal strength)

Notice how as I drag it towards a color, it becomes "weaker".
You can add other colors to a gradient, it'd probably be easier to use the Gradient Panel (Window → Gradient or Ctrl+F9)

Just drag and drop a color into the gradient to add it:

Welz is dead-on about the gradient panel.
Referring strictly to the Gradient Annotator which appears on gradient-filled objects on the artboard when you hover over them with the Gradient Tool..... Each "bit" has a different function. Some are similar to the panel functions, some are additional or different...

- Black Circle on the left - click-drag to reposition/move the gradient.
- Black Diamond on the right - click-drag to change the length of the gradient or hover the cursor just outside the diamond and you can rotate the gradient (you'll see the cursor change to the "rotate" cursor, then click-drag). The center of rotation is the black circle on the left.
- White Squares are "mix" points. You can use these to alter how two color stops react to one another, allowing one color to extend closer to another. By default, these squares are placed at the 50% point between two color stops.
- Color Pointed Squares control the color and opacity of that stop point.
You can double-click a color stop to alter the color of that stop. Or merely click the bottom edge of the gradient "bar" to add a new color stop at the click location.