Paragon Lost
Terminally Lost
GURPS went all in on the toolkit approach. That's fine and all, but it meant that they had to account for the possibility that one side might have a guy with a heavy crossbow whereas the guy on the other side had a Kalashnikov RPL-20.
One of the reasons many people didn't like introducing firearms into fantasy games is that systems rarely had a method to model what made a modern firearm superior to the ancient stuff. Firearms usually end up feeling like reskinned wands. The 1-second round fixes that, but in a fairly unsatisfying way (gaming wise) as it makes anything "slow" generally not fun to use. And if you aren't using firearms, you don't need that 1-second round at all (it's no longer solving anything but the slow stuff is still not fun).
Here's how I accounted for it back in the late 1980s and into the 1990s when it cropped up occasionally. "No". No fuss, no muss and very little thought required. If I felt expansive, I'd say, "no, modern day weapons are not in my current campaign. I've given you the races, advantages, disadvantages and skills list along with equipment that you can start off with". Feels like your making it harder than it has to be.
edit: The whole hex or square map debate is interesting but I don't get the issues. I preferred to use hex maps versus square even before GURPS. (shrug). Again, feels like there some here who are over thinking it and making the use of hex or square harder than it has to be.