My 7 yr old Lab/Sheperd cross is coughing so badly and gagging and barfing due to the phlem. It's been going on for about two days now. Can I give him "buckleys" for chest congestion to help relieve his symptoms.

No.
Don't ever give your dog any human medicine without speaking with a vet first. The cause of the coughing should be pinpointed first.

I will echo Sariss...
Human medication can be very harmful, even fatal to dogs.

Always consult your vet and never self medicate.
It could be something like kennel cough which will require antibiotics, not cough medicine.