Árni Leifsson and Andrew R (and possibly others) have pointed out that in the word ‘gymbr,’ the /-r/ ending is actually part of the stem! Seems I need to brush up on ON morphology a bit. However, I still don’t think this detracts from the point (perhaps it even makes it more cohesive) – Old English speakers might have kept it here because they recognised that it wasn’t an inflectional ending.