The military is also not the only protection system in place. You are right, just considering the GDP is too limited, but considering just the military is also too limited. Police, fireman, courts, security systems, the SEC, locks, alarms, etc all contribute to protecting assets in the US.
It's a hard number to measure. Bitcoin inflation has a fixed measurable cost though (12.5 coins per 10 minutes), if that's acceptable to you for the benefits you get, then it's money well spent burning electricity.