Q: High speed acceleration issue

Yes, you do have an old truck, but that doesn’t mean it can’t run well. If you noticed this problem about the time you replaced the catalyst, it could be that the replacement catalyst is not large enough to breathe well. Or if the old catalyst failed, it could be that you have debris from the old catalyst jammed up in the muffler. Otherwise, the first thing I would do is to be sure all the service items are up to date. Plugs, filters, etc can go a long ways. Your truck does not have sophisticated self-diagnostics, so you probably won’t get much from that. It will have to be checked out old-school style. You can have that done at your convenience by contacting YourMechanic. They can send a technician to your home or office to check out your GMC and let you know what it will take to solve your problem.