Approximately, yeah. But the formula for how much a guy helps your ERA is a little more complicated than that.
After all, if a guy has a 2.00 and 80 innings, he'll help your ERA, but if he has a 4.00 ERA and 160 innings, he'll hurt your ERA.
Basically, though, Gagne had enough innings to be significant, and a very, very low ERA and WHIP. If a guy had 5 innings and a 0.0 ERA and WHIP, it wouldn't do all that much for you, but once it gets to about 40 innings or more, every little bit helps, assuming his stats are good enough.
(taps little picture of a microphone)... is this thing on?