Well, I don't know if it can be accomplished easily using a cheat or something. At any rate, it can be done by hacking all the singleplayer .bsp files (there are about 25 of them). If you're up for the challenge (it certainly is a bit tedious), I can tell you how. How badly do you want to change the gravity, anyway?
If you were prepared to install a Perl interpreter, I could post a bunch of scripts that would allow you to edit the entities lump in a .bsp file (which is exactly what you need to change gravity) without much effort.
Well, here you go:
Included is a shell script addgrav.cmd that does exactly what you want.
E.g., if you want to set the gravity in a bunch of sp maps to 700, extract the respective bsp files from assets0.pk3/maps into a new folder called maps, open a command prompt in that folder and enter the following
for %f in (*.bsp) do addgrav %f 700
(provided perl and addgrav.cmd are lying in your PATH).
Then pack the new folder up in a pk3.
Uhm, one more thing: if you're intending to try it out, please note that you do so at your own risk (read the disclaimer in the details section on the filefront page beforehand). I cannot be held responsible should the scripts prove defective and have unexpected side effects
(I've tried them out on all vanilla sp maps and they worked flawlessly, but I haven't thoroughly tested them, to be honest).