The 50m fly is not considered one of the Southport star's strongest events, and defeating both Guehrer and Seebohm is a surprise result for the 20-year-old.
Halsall helped herself to an excellent start in her 50m butterfly final, but only just held on for gold ahead of Australian duo Guehrer and Emily Seebohm.
But the 22-year-old picked herself up to win silver in the 50m backstroke event later on Friday, finishing just 0.03 seconds behind Australia's Sophie Edington, while Davies and Emily Seebohm of Australia could not be separated for bronze.