Identical: similar or alike in every way EX: The two cars are identical
except for their license plates. Reference: dictionary.com
Im not saying their twins... which for the purposes of the argument are not the same person but are referred to as being identical because of their obvious similarities.
-First of all Gotz is quite obviously similar to Guts in name.
-The iron prosthetic arm which you also already stated is very similar to Guts's canon arm. "His hand was ahead of its time, being capable of holding objects from a sword to a feather pen." reference: wikipedia
-->canon arm that can hold a sword
-As you mentioned they were both mercenaries which shouldn't be ignored, but another similarity is a play based on his character which fabricated the following description of the struggle of Gotz in which he is referred to as, "Götz von Berlichingen, a noble knight with an iron hand, is hated by the rich and powerful and loved by the oppressed and poor." reference:
http://www.jrank.org/literature/page...n-Hand%29.html
-->This struggle could easily be construed as the epic and current struggle by Guts in his revenge against Griffith. (a little bit of a stretch but worth noting)
-"In 1525, with the outbreak of the Peasants' War, Berlichingen led the rebels in the district of Odenwald against the Ecclesiastical Princes of the Holy Roman Empire. Despite this, he was (according to his own account) not a fervent supporter of their cause. He agreed to lead the rebels partly because he had no other option, and partly in an effort to curb the excesses of the rebellion. Despite his wishes to stop wanton violence, Berlichingen found himself powerless to control the rebels and after a month of nominal leadership he deserted his command and returned to the Schloss Jagsthausen to sit out the rest of the rebellion." reference: wikipedia
-->Guts didn't support Griffith's "dream" and wanted to find his own and left the band of the hawk.
-The time in which Gotz lived is similar to the time in which Berserk takes place (c. 1480 – 23 July 1562) reference: wikipedia (the weapons and armor are similar and there is a distinct lack of technology)
Did Guts fight with Farnese as a holy warrior... i don't really recall or care to reread that section. But the fact that Miura put in aspect of a holy army shouldn't be written off as, "I can't see Guts fighting for the Pope now."