I do not believe Google's mail servers reject email which fails an SPF check. If that is the case then implementing SPF for your domain will not stop the backscatter from Google. It also won't stop backscatter from any system which does not support SPF at all or any system which only uses SPF to score spam.

SPF was a good attempt at stopping email forgeries. It has its flaws and its usefulness is hampered by limited adoption by MX servers.