Which gerund? 'A-musing'? That is an old-style form meaning 'thinking': she fell a-musing = she began thinking.
The Milk-Woman and Her Pail
A FARMER'S daughter was carrying her pail of milk from the field to the farmhouse, when she fell a-musing. "The money for which this milk will be sold will buy at least three hundred eggs. The eggs, allowing for all mishaps