Absolutely no. GC's don't cause leading, nor do they actually prevent it, and they certainly don't check gas blow by to do it. If you'll read my book Jacketed Performance with Cast bullets, you'll get a detailed explanation of all this.
At mild velocities where leading isn't a concern the copper is running in bullet lubricant, which prevents copper fouling. At higher velocities/pressures, where the bullets are leading, the gas checks scrape up the melted lead from the bore, which in turn displaces the lubricant forward of the check, which provides the gc with both grease and lead as a lubricant. (Lead is one of the highest pressure lubricants there is.) If gc's aren't keeping the barrel clean, and one recovers a few fired bullets he will find the check shank space forward of the check will be full of lead which stops the scraping action and allows a thin smear of lead on the bore, though not usually detrimental, if pressure and velocity aren't insanely over a reasonable speed.
