GPI means General Paralysis of the Insane
If he died in 1911 I would have thought he caught it after the death of his wife as untreated it's a quick death
This is worth reading
goo.gl/cGBYSc
Also Wiki gives a very good timeline
goo.gl/sBFq3Q
Sorry but this is incorrect.
Syphilis has 3 active stages, plus a latent stage during which time the infection is dormant. The primary stage is the chancre -- the painless ulcer which may go unnoticed if it's not in an area that sees the light of day much. That comes up anything up to 3 months after contact, and is infectious.
The secondary phase is very infectious; it's when the bacterium spreads around the body I foret how quickly that phase can begin but it's usually a few weeks/months after the primary chancre heals. I've seen people with active secondary syphilis a year after their first contact.
The bacterium then lies dormant unless and until tertiary syphilis can develop. This affects the heart or the central nervous system (or if you're particularly unlucky, both!) In the central nervous system, it can cause tabes dorsalis, which destroys the nerves to the feet producing numbness and a classical gait; and it can cause GPI, which was not a paralysis at all -- think of it more like a type of advanced dementia. Tertiary syphilis can rear its head anything up to 30 years after the primary infection.
So OP, I think that it's quite likely that he picked up the infection some time around the time that he married (and remember, he could have caught it from your gt-grannie rather than vice versa).
The list of child deaths is heartbreaking and even more so when you remember that it wasn't that uncommon, but remember that children died of many more things than congenital syphilis
and in fact it's not unusual at all for children to survive congenital syphilis and live into adulthood (they cannot pass the infection on though).