The ‘garth’ in Shetland placenames I believe means farm or enclosure, derived from the Old Norse garðr. The ‘muckle’ I imagine has come from mikill in Old Norse, meaning big or great (‘Muckle’ is also used in everyday modern Shetlandic, meaning large). So perhaps there was a large farm here.
Interestingly, in the Játvarðar Saga the city of Constantinople is called Micklegarth, referring to the big wall or stronghold.
[Gulberwick during Lockdown collection - Image 20 of 80]