First, make sure you have the complete current Sound Library installed -- GarageBand menu > Sound Library > Download All Available Sounds [AFAIK GB only installs a basic sound set in the initial installation]. Once you've done that check the expected categories -- Strings, Orchestral, Legacy [if there], Jam Pack [if there], etc.. Apparently the iOS version of GarageBand has Cello sounds, so I'd assume they're probably also part of the full GB Library in a macOS installation as well, though I can't say for sure.. I don't think I hav the entire GB Library installed here, so, again, I can't be certain what strings are currently included. On my system I see a few string [ensemble] Patches in the Orchestral section in the Library -- if you play in the cello range on the String Ensemble patch, for example, you'll probably get the sound of a cello section, though it may be mixed with other strings. I also see a Smart String patch, but that doesn't seem to extend below the Viola range. I have some other string patches in the Legacy and Jam Pack categories, but I think those are from my Logic installation, I don't know if they come with the full GB Sound Library installation. Of course if you can't find the cello sound you're looking for [ensemble? solo cello?] in the GB Library you can always turn to third-party instruments, both free ones and commercial sound sets, like from Native Instruments, Spitfire Audio, Big Fish, etc -- just search for 'virtual instruments cello'. GB would require the instruments to be in the Audio Unit format, but commercial libraries should always include that format.