All the -gress words are related through their Latin roots:
progress: (verb) to go forward, advance; (noun) forward movement
regress: (verb) to go back
ingress: (noun) entrance
egress: (verb) to go or come out; (noun) exit
congress: (noun) the act of coming together and meeting
retrogress: (verb) to move backward, revert
In all of them, the “gress” part has the movement—the going—and the prefix indicates direction of movement.
Aggression is also related, meaning attack—going at. There are probably others I’m forgetting.