He has signed for an initial month-long loan from Northern Counties East League side Handsworth FC, where last season he was fast-tracked into the first-team squad and scored nine goals in 19 appearances.
Howarth, known for his pace and clinical finishing, has netted 10 times in 17 appearances this campaign.
He said: "I'm blessed to have an opportunity to play for a club like Gainsborough.
“I've heard all great things about this club and I want to bring my abilities along with me to ensure this team goes forward.
“I hope my time at this club brings joy to myself and everyone associated with the club. I can't wait to get going."
Manager Woodhouse added: "Leon is a player with lots of ability and potential.
“He has great feet and a great turn of foot, and is also an accomplished finisher and will be a good addition to our squad.
“We would like to place on record our thanks to Handsworth FC for helping us get this one over the line so.”