How to use GCM mode and associated-data properly
GCM mode already incorporates any params that could affect the outcome of the decryption. The associated authenticated data is there to allow you to rely on context for your decryption.

For example, say you are encrypting some records associated with a user. You may want to include the user's database ID as the authenticated data. If a user found a way to copy another user's data and key into his own record, it would still fail to decrypt since his database ID is not the same.

In general, you would like it to be something that's inherently managed separately from the nonce and ciphertext.