Got my great-grandparents' marriage certificate in the mail! Here's a link to the PDF.
From the marriage certificate, I (supposedly) now have the names of all four great-great grandparents, but the handwriting is really tough. Here are all the details from this doc, according to my initial attempt to figure it out:
Name: Charles Gordon
Address: 137 Suffolk St, NY, NY
Birthplace: Russia (Was hoping for a town, but no such luck)
Father's name: Israel (no last name given, so do I assume "Gordon" is original?
Mother's maiden name: Esther Ossofsky ? Or something like that...
Name: Annie Cohn
Address: 127 hindlow st. (almost certain I'm reading that wrong)
Father's name: Tevew (Probably reading that wrong, too)
Mother's maiden name: Chaie Clursky ???
Ceremony performed by: Mendel Sampro??
Horodisclie??? Bros Ben??? As???
109 Cudlow St??
Witnesses: Mosel Kaplan and Sam Meir?
The next thing I'll do is focus on trying to track down Israel Gordon. If the family stuck with Jewish tradition, my grandfather would have been named Israel, too, rather than Isadore. And I did find a NYC birth certificate for an Israel Gordon for his correct birthdate. Nothing for Isadore. Would they have named hims Israel and then changed it for some reason? Not sure.
The name "Gordon" is original to parts of Russia, especially an area called Grodno in what is or was Belarus. So I'll start focusing there.
Also ordered Charles Gordon's death certificate, which hasn't arrived yet. But comparing the handwriting on his parents' names may help figure out some of the spelling.