I;m sure I have seen this post on the forum before but anyway...
The GC will have issued you with a Statement of the Offence.
The Statement includes, at the bottom of the form, details of the officers identification and the cuartel from which they operate.
On the reverse of the side of the form are details of how to pay the fine, how to dispute the fine and how to complain.
Also check out this link dgt.es for the same information.
BTW marked guardia civil vehicles all have number plates including the letters PGC.
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