You asked an interesting question, so I decided to research myself, I figured it would be a good addition to my mapping guide.
When two environment fields are placed, their gravity mods are multiplied, like you guessed. So if a 5x gravity and a 0.1x gravity field overlap, you will have 0.5 gravity.
So this could cause a problem if you were hoping to have no gravity in space, since 0 times any number is 0. You could help fix the problem by making the gravity in space 0.01, and then making the gravity on the Earth sphere 100.
If you need the gravity mods for each body in our solar system, I copied this table off of Wikipedia:
Name Magnitude
The Sun 27.9
Mercury 0.37
Venus 0.88
Earth 1.00
Moon 0.16
Mars 0.38
Jupiter 2.64
Io 0.18
Europa 0.13
Ganymede 0.14
Callisto 0.12
Saturn 1.15
Urstar fish 0.93
Neptune 1.22
Pluto 0.06
As for making the sun visible, either use the sun fx object, or use a large static object if you want people to actually be able to fly to the sun. You should probably put a huge kill trigger around it if you use that method.
If you use the fx object, you can change the radius and brightness of the sun fairly easily.
Oh, and vehicles aren't effected by gravity mods, so you'll be able to drive your jeep off of the side of the Earth and fall into infinity.