Hosting a national TV show where he performed music didn't hurt.
That didn't happen until he already had a couple of huge hits under his belt along with no less than 7 successful albums, though (including a Christmas LP and a duet album with Bobbie Gentry). While his TV show certainly bolstered his popularity, he was already undeniably a massive star even a year before the Goodtime Hour premiered.
Wasn't he already a major country star when he got his own show, though? It's hard to imagine his label thinking about dropping him at that point.
Yes, he absolutely was. A major pop star, too, with several huge crossover hits to his name. Goodtime didn't premiere on TV until January of '69 (he had hosted the summer TV replacement for Smothers Brothers that previous summer).
That "experiment" with Al De Lory I mentioned (when Capitol was thinking about dropping him) took place in late '66 and early '67. The single "Burning Bridges" was the result and though it missed the pop chart, it performed well enough over on the country chart that Capitol decided to keep him.
By the time he got his TV show, he already had hits with "Gentle on my Mind," "By the Time I Get To Phoenix," and the sublime "Wichita Lineman," a #3 pop crossover smash. Even before his filling in for Smothers Brothers in the summer of '68, "Gentle" and "Phoenix" were hits.
Gentle was a top-5 album,
Phoenix top-15. '68 was a
huge year for Glen, and while he likely did get a boost from filling in for the Smothers show, it wasn't his own show at that point.
Edit: Oh, and I almost forgot... Glen also co-starred alongside John Wayne in 1969's "True Grit" movie. It premiered in June 1969, which means Campbell was almost certainly chosen for the part before January of that year when "Goodtime" premiered on TV. Great film, too!