Export Volume Low in iTunes?

Try this…

Double check that you have an overall, adequate output level from GarageBand itself, from the main out on the lower right. Output to your headphones could be different (being monitors) than the actual exporting of the file, and might be giving you a false sense of loudness.
Second, check and uncheck Normalize in your GarageBand preferences and compare exports between the 2. When checked, it can sometimes reduce the volume of the whole master track in relation to a random, undetectable volume peak.
Also (this one was caught by musician, Eric Flowers), see if you have “Sound Check” selected in iTunes. Uncheck and see how it sounds.