Re: Colonizing other places in ours sytem

Re: Retaining an atmosphere.
If you can _get_ an atmosphere back onto Mars, the question is of how 
long you can retain it. If I remember correctly, If you instantly gave 
a ~ 1 bar earthlike atmosphere to Mars, it would continually thin but 
remain liveable for hundreds of thousands of years without active 
maintenance. The time would be considerable shorter (order(s)(?) of 
magnitude) for the moon.

Re: human physical adaptation.
As has been said, as long as the trip were one way, the loss to bone 
density etc, might not be considered a major issue. It would affect the 
absolute physical durability of long-time residents or natives of the 
low gravity environment. This has been fodder in the gaming and SF 
worlds forever, where high-g worlders tend to be stronger and hardier, 
sometimes awkward at low g, and low-g'ers more frail and "quick", 
sometimes crippled at high-g. Has this been folded into SG or DS yet?

Re: immune system problems.
If they're real, positing development of immune system boosters 
(biological or otherwise) in the not-too-distant future is no further a 
stretch of imagination at this point as positing a self sufficient 
colony on another world. Certainly more plausible than gravity plating.

Related GZGverse question: what human-colonized planets have the 
strongest gravity? Could/should they be considered the best locations 
for (physical) training of ground troops (assuming orbital facilities as

well for low/0-g components?

Big 'n' Mean Zero  (Noam Izenberg  - Don't mess with this Null)