Megan, I've seen so many different definitions for conditions where you might experience engine icing over the years, many of which specify either visible moisture or visibility less than 1000m below anything from +10 to 0.
The Gazelle was used regularly for instrument flying, including airframe icing conditions (when the FM aerials began to wave around it was time to leave) - never had an engine anti ice switch or facility and never, to my knowledge, experienced engine icing.