I thought the authors misused the word 'slowing', because it seems that 'slowly' as an adverb should be used. However, there are multiple examples using slowing + gaining + something(noun). Really appreciate if you can help explain the use of 'slowing' in these examples.
Offering a more optimistic view, several Federal Reserve officials gave separate speeches yesterday arguing that the economy was slowing gaining strength. -The New York Times
... the season standings by finishing no lower than second 10 times.Hamilton chased Rosberg for 23 laps, steadily keeping up the pressure and slowing gaining on him. Hamilton finally made his move on lap 24, squeezing inside Rosberg's … -Los Angeles Times
... on February 13, the JTWC upgraded Feliska into a tropical storm as it turned north.While slowing gaining strength, Feliska turned east. By 0000 UTC February 16, MFR declared that Feliska attained peak intensity. … -Wikipedia