Daf 65A in Zevachim uses the words "Derivable from" followed by a gezera shava. But "Derivable from" is a phrase usually followed by kal vchomer. Here it is followed by gezera shava. But a gezera shava is learned by tradition not by logic. Is it saying the gezera shava here is logically derived?
If you are referring to the Gezeirah Shavah of "u'Malak-u'Malak," note that the Shitah Mekubetzes (#6) takes out the words "v'Din Hu," precisely for the reason that you point out.
D. Zupnik