Hey MW,
Just googled to see if there's a 5G menu online. There is & there are 17 toppings, including sliced tomato, cheese, bacon, jalapenos, grilled onions, grilled mushrooms, etc. One just opened in Pt. Loma, so guess I'll have to sacrifice the extra calories to see if there are any California-specific toppings

I read an article a while ago about Harris Ranch (may have read it while at the restaurant). Apparently, I&O gets their chuck beef from Harris Ranch to grind for their burgers. So I'd say I&O probably has the edge on quality.
But 5G uses good quality beef, toasts their bun on the grill, uses two patties & has the grilled onions and/or mushrooms option. Plus, the fries are either plain or cajun. The way they grill the burgers to order, it's more like a diner burger than I&O.
If I were to guess, bc of all the similarities (red & white tile decor, open kitchen, peanut oil, fresh cut fries, basic menu), I'd say 5G evolved out of several visits to I&O w an effort to improve the template.