Gintama isn't the highest selling title, but it is one of the higher selling titles. It can comfortably outsell most of Jump's offerings, even if the sales have declined somewhat from their glory days where it would easily get 600,000 a volume.
One Piece pretty much towers over everything, but Gintama is pretty solidly in the second tier of Jump's properties when it comes to sales. It's always been an extremely strong performer in the TOC rankings and in sales.
As far as them letting the show go idle, well most new properties do that nowadays. The days of a show just doing endless filler or bad pacing to stay on the air has mostly been reduced to legacy titles (and I'm not sure why someone would want that for a favored title). Toriko is following in that path, but most new shows of long running manga now just go seasonal, even when they're extremely popular (look at Kuroko which is one of the most popular series out there right now).
In fact, I would actually say that Gintama's anime paints an extremely good picture. It's yet to miss airing in a single year, even as they adopt strange schedules. To me, that shows the high level of popularity the series has, and the fact that they're always willing to bring it back when they have more material to animate. History is lined with anime which took a break because they got too close to the manga and then never returned, or maybe had one extra season but had to wait years for it. Gintama has probably gotten the absolute best deal out of just about any property when it comes to treatment in anime form. It shows just how massively popular the series is in both manga and anime form.
