Green’s Dictionary of Slang

a euph. for God, and as such used in mild oaths.

1805		J. Reynolds Blind Bargain II ii: By Gom! – I’ll make them stand in white sheet – I knew it all along.
1815		‘Song’ Sailor’s Vocal Repository 28: I seized upon madam, by gom she were fuddled.
1856		in J. Strang Glasgow and Its Clubs 585: By gom! that’s an ill-faured neebour.
1946		K.S. Prichard Roaring Nineties 139: By goom, if they come samplin’ reef of mine, I’d bring out me bluderbus.