Two problems. GPS is not real useful (except for "mid course guidance perhaps), when the targets can move. GPS doesn't always work well inside a building, let alone under salt water. Those missiles and torpedoes need some kind of active or passive seeker head, which they of course have.
“Those missiles and torpedoes need some kind of active or passive seeker head, which they of course have.”
Inertial guidance is pretty good. In the sixties my ship left Mayport Florida did a 9 month Med cruise and docked at a different dock when it got back. The SINS system knew all about it. This was WAY before GPS.
The GPS info is loaded into the Harpoon, then when it reaches target proximity, it switches on its seeker
function. This enables the firing platform to engage other targets without continuous tracking after launch.
http://www.fas.org/man/dod-101/sys/smart/agm-84.htm