Issue Updating Google Apps - "Not Signed Correctly"
In Android, every package is cryptographically signed by its author. To be an upgrade, the new package has to be signed with the same key as the old package. Since packages can read previous versions' data files, this is an important security precaution.

The error you get tells you that the update you're trying to install was not signed using the same key as the package you had flashed to the ROM. Either the original package you flashed, or the new package, was signed by a different developer.

You should either obtain an update from the same source you originally got the gapps package from, or flash a new ROM with a gapps package from the same source you're trying to update from. We may be able to help further with this if you edit your question to say where you got these packages from.