Here is the article by Tim Wiles cited in the Pittsburgh Post-Gazette article:
http://baseballhall.org/discover/honus-is-on-you
Wiles gets some details wrong -- dating the American Card Catalog to 1937, citing the $50 valuation for T206 Wagner when that was only in the 1953 and 1960 editions of the Catalog. I assume he got the Gruber-Wagner story from the book The Pittsburgh Pirates, which was written by sportwriter Fred Lieb and published in 1948. Lieb had been a sportswriter since 1911 and presumably knew Gruber, plus Honus Wagner was still alive at the time, so I'm inclined to believe the story even in the absence of any more contemporary source. Also, the story is broadly consistent with the story that Honus Wagner told John D. Wagner in 1941, as I just posted in Leon's thread on the T206 Wagner:
http://net54baseball.com/showthread.php?t=241057