My dog had gas, now screaming, can't walk. Should I see the vet?

Oh no. Poor Rocky. He may benefit from anti-gas over the counter medicine if that's the case. Since we haven't seen him in person and actually examined him, we can't recommend a particular medicine to use, but your vet should be able to recommend the appropriate one for him and his size.
You can also try gently massaging his belly to move the gas around. For the future you should consider placing him on an easily digestible food such Iams limited ingredient or the Innova (Evo) brand for sensitive stomach.

However, I am a little suspicious that this may not be all that is going on with him, especially if he is as painful as you describe or doesn't respond to the anti-gas therapy. It may be beneficial to have the vet take another look at him to ensure there is nothing else going on there. I hope Rocky feels better soon.