Keep in mind that GarageBand is a stripped-down consumer version of Logic Pro X. A lot of features are not available in GarageBand because they are considered (by Apple) as more professional features. That includes the feature you are talking about. In Logic, it is called "Selection-based Processing" (in Pro Tools, it is called AudioSuite).

The Selection-based Processing feature lets you apply a complete set of Audio FX (like on a Channel Strip directly to an Audio Region or a section of a Region. This will let you apply, like in your situation, a gain offset or filtering to those section with noise, pops, etc.
If you are experiencing situations where you hit the wall with GarageBand and you need more advanced features for your productions, then maybe it is time that you "outgrew" GarageBand and time to upgrade to Logic Pro X.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'