
By gum!
Regional Flavor
Mainly used in Northern England, 'by gum!' adds local color to your English and can show familiarity with British regional speech. 
Archaic Charm
'By gum!' is old-fashioned, so using it can give a vintage or humorous tone to your speech. 
Expressive Exclamation
Use 'by gum!' to express strong emotions in a mild, non-offensive way. 