The problem in evaluating Gallardo so far this season is that we're dealing with a small sample size, which is almost never indicative of actual performance.
If we go back in to last season, since just after the All-Star Break, his ERA is 5.77. So far this season, his ERA is 4.62. His career ERA is 3.72.
However, we can't look at ERA alone as an indicator of performance. The more concerning stat is his strikes per nine innings (K/9)
Before the All-Star break, his K/9 was 10.05. After, it was 9.33. This season so far its 4.62, meaning he's not striking people out this season. In his complete game shutout he struck out just 2 batters.
Clearly he was dealing on all cylinders in that complete game, so what happened afterward? He did get an extra day's rest due to the rainout, wihch may have messed with his schedule. Pitchers are creatures of habit.
Manager Ron Roenicke said he wasn't worried, but maybe Brewers fans should be. It wasn't an obvious problem, like Axford's struggling for control, that led to Gallardo getting lit up for seven runs. Until the Brewers can nail down the cause of the poor pitching, we could be in for a few more short starts. And the already beleaguered bullpen can't really handle that right now.