The Samsung Galaxy Gio cellphone ships with Android 2.2.1 (Froyo). To take a screenshot on this phone, touch the Back button (at the right), keeping it touched press the Home button (in the center), release the Home button and then release the Back button.
You should see the message: Screen captured. Saves as image file. You can find the screenshot images using My Files stored under the directory /sdcard/ScreenCapture
Tried with: Samsung Galaxy Gio running Android 2.2.1 (Froyo)