In America the GFCI will be rated to trip at 5mA differential. In Europe it is 30mA, in the UK we refer to the devices as RCDs.
A blog on element14 found a similar issue with a GFI plug and dismantled it.
https://www.element14.com/community/people/jw0752/blog/2017/12/23/reverse-engineering-a-gfi-plugThere are some links to information on RCDs and GFIs in my reply to that blog.
In the UK, recommended testing is 3 monthly using the built in test button and they should be fully tested during initial installation verification and subsequent periodic inspections, which are 10 years for domestic and 3 to 5 for commercial / industrial.
RCDs are not the most reliable of devices, up to a 30% failure rate is not uncommon but it is dependent upon the ambient conditions they are installed in. In the UK, they are regarded as supplementary protection for a circuit but are now mandatory for the majority of new installations or alterations to existing installations.
As to whether or not on would trip if a finger shorted out a lamp holder, I am not totally sure. It will depend upon the leakage from the finger down through the rest of the body to earth in comparison to the current through the finger from live back to neutral. In theory, the current flowing through live and neutral will be the same, so the RCD will not detect that, but the current may exceed the overload rating of the circuit breaker which is likely to be 6A in the UK.
If no overload is detected, then the RCD would detect the leakage current from live, through the body and down to earth and should be enough to trip either a 5mA or a 30mA rated device.
As already stated though, lets leave this one in the theory department and not do any practical tests.
Kind regards