That sounds like a fairly generic analog synth pad with a slow attack/release and heavy filtering with a lowpass filter. It's unlikely you'll find the exact sound in GarageBand [or any synth, unless you have reason to believe the composer used GB and/or used a factory patch] -- you'll probably have to program it yourself with an analog synth V.I., or tweak a similar-sounding patch -- maybe a basic string pad -- to match it as best you can. [Keep in mind there may be layered sounds/parts in there -- it may not be just a single patch]
GB is not especially well-suited for tweaking synth sounds -- you only get a limited number of controls in the Smart Controls panel, and you can't open the built-in Instruments' plug-in windows to get at the full range of programming controls. However if you have a third-party analog synth Instrument installed, you should be able to open that Instrument's plug-in window.
If I was going to program a sound like that from scratch, I'd start with layered/detuned sawtooth waves, a resonant LPF, and slow settings on the ADSRs, and then maybe add some additional processing (chorus, reverb). If you want to learn more about synth programming , check out some of the [analog] synthesizer courses on this page:
For synthesizer basics, check out this Audiopedia course: