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davidcyclebackMost real photo postcards were not issued/sold to the public. Many are simply family photos. Though there are cases of real photo postcards sold as commercial products, most notably 1930s Hollywood movie star postcards that were available in many stores.
Even if you don't know exactly how and who issued it, a postcard that was machine printed is generally considered to be a commercial issue. This is because lithography, photoengraving and other were mass production processes for commercial projects, and wouldn't be used to make a family photo for the mantle.
As far as the Gibson postcard goes, someone would have to convince me that it was or probably was a commercial product before I considered it a baseball card. I just read the PSA article. That this Gibson and other postcards originate from the personal collections of contemporary Negro League players would suggest they weren't commercial issues. If, instead, they were found in the scrapbooks of Joe Schmoe fans, then they'd appear more likely to be commercial issues.