> space is known to be full of atmospheres to use to slow down

The places people talk about travelling to tend to have atmospheres and gravity, yes.

How many G's of deceleration are we talking here? I imagine even 2Gs of force wouldn't be tolerable for humans for more than an hour

> even 2Gs of force wouldn't be tolerable for humans for more than an hour

You can tolerate 2G for hours, particularly if everyone's oriented eyeballs in. That said, both aerobraking and gravity assists are intermittent accleerations.

so you've been traveling at some large speed for quite some time, and you are now proposing to come to a near stop to come into orbit around some far away planet? and what magical tech have you forgotten to tell us about that allows that sudden deceleration to not liquefy the bags of meat inside the ship?