While creating sliding 808s in Garageband is possible with the Deep Sub Bass Synth, it's much better to use third-party plug-ins.
You can use parallel compression in Garageband in two different ways.
How to install plug-ins in Garageband iOS, including a list of free and paid software you can try.
A list of some of the best and most useful plug-ins for vocals, including mixing, editing, and FX plug-ins.
How to make a country song in Garageband. A step-by-step guide, as well as a brief discussion on music theory.
How to make a metal song in Garageband, including necessary gear, plug-ins, tips, tricks, and a step-by-step guide.