Sorry this isn't actually answering your question but I too wanted to upgrade to 5.1.3 and ran in to the problem you have. If I run the 5.1.2 Product Compatibility Checker it reports no incompatible products. McAfee Gold support were not able to give me any further information, so I've been left to presume, like you, that 5.1.3 does not support any patch level of GTi Proxy.
Looking at the Supported Platforms data for GTi Proxy ePO 5.1.3 does not support GTI, maybe related to the EOL announcement where GTI is End Of Life from Dec 31 2015.
Certified McAfee Product Specialist - ePO
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