Hi -
GarageBand doesn't have a dedicated mastering Limiter like Logic's AdLimiter, but the regular Limiter plug-in in GB should be useful. Display the Master Track, open up the Smart Controls Pane (B), and insert the Limiter at the end (the bottom) of the insert chain of Master Track plug-ins. Set the Output Level (Ceiling) to -1dB and crank up the Gain to increase the average level. Unfortunately GB doesn't provide the kind of metering tools you'd need to guage the actual average level (or the LUFS level, for streaming applications) -- for that you'd need Logic or some third-party mastering plug-ins..