It would have changed the dynamics massively if Gary and Val had stuck around for a long length of time. Gary's presence would mean loads of upheavel, like influencing Bobby in some ways, scaring J.R. with his potential, Gary doing some things brilliantly because he wanted to, Gary running away under pressure, getting drunk worse than Sue Ellen at her worst, going on a rampage, affairs etc. Val is also so stubborn, someone you can never truly shake off if she's annoyed and sticks around. She would be fretting when Gary does things wrong or cracks under pressure.
They would fracture a lot of the family dynamic. Gary and Val's issues would effect the rest of the family.