Hi,
GarageBand offers limited access to synth parameters so that can be difficult to achieve, unless you are using a 3rd-party synth plugin.
If you are only using GarageBand's built-in sounds, try to load the Screamlead preset (under Synthesizer > Lead > ...). You will be able to tweak the Glide parameter under Controls to change the glide time.
I hope it helps!
Christian