Error:
Signature error - Got xxxxxx, expecting yyyyyyyy
Visible on the Gateway Log.

Cause:
SERVER TIME - A difference in the server time clock also triggers this error.

HMAC KEY - This error has been reported on some systems where the hash_hmac or bas64_encode PHP functions behave differently than expected. We suspect tildes or special characters on the HMAC Key trigger this error.
We have yet to pin point the exact circumstances, operating systems, and/or PHP versions affected by this.
Reported on these systems:
-CentOS 6.5 x86 64 with VMWARE with PHP 5.4.23

Solution:
SERVER TIME - Correctly set the server time clock. (Sometimes you have to contact the Hosting provider because the clock is set at the hardware level)
HMAC KEY - No solution at this point. Only a workaround is available.

Workaround [HMAC KEY]:
Go to your First Data Terminal API Settings and generate and update to a new HMAC Key that has no tildes (avoid underscores also, if possible, just to be safe). Update your gateway settings with the new HMAC Key.