You need a software instrument bass that supports different expressions. There are some types of bass in the Jam Pack 3: Rhythm Section Instruments. These instruments support slides and other types of expression using the mod wheel or the velocity. Modulation and velocity can be controlled in the midi draw area of the GarageBand editor window.
The JamPack instruments do not come with GarageBand. You can install them by purchasing MainStage 3.
For example, if you use the Attitude Bass, Glam Rock Bass, Grind Bass:
Aggressive Fretless: Fretless electric bass with strong bite and attack
• Mod wheel adds vibrato.
• Highest velocity adds slide down to note. Attitude Bass, Glam Rock Bass, Grind Bass: Fretless electric bass played with a pick
for heavier styles such as alternative rock, punk, and metal.
• Mod wheel adds slide up to note. • Higher velocity increases attack and intensity. Lowest velocity plays stopped notes. • Notes below E0 play slides and dives.
Liverpool Bass: Electric bass with a round, lyrical tone for classic rock and melodic pop.
• Mod wheel makes the sound more muted. • Highest velocity adds a slide down from the note. • Notes below E0 give slides, harmonics, and pick sound.
Motown Bass: Deep, warm electric bass reminiscent of the classic Motown sound
• Mod wheel makes the sound more muted. • Highest velocity adds a slide down from the note. • Notes below E0 give slides, harmonics, and pick sound.