The 18-year-old - whose father, Keith, was a prolific goalscorer for Gateshead during the 1980s - was born in Newcastle and attended Emmanuel College in Gateshead.
McNall has scored five goals in three starts for United's under-23s this term, having been sidelined through injury at the beginning of the campaign. He was a prolific scorer at under-18 level in the previous two seasons.
He has signed until the end of the season and could go straight into Steve Watson's side for Saturday's Vanarama National League trip to Chester City.
Former Newcastle player Watson, who was appointed Gateshead manager in October 2017, told his club's official website: "You never know who you can unearth by giving young players a chance at senior football and if you know where the goal is and you're composed in front of it then it doesn't matter what level you are at. "Hopefully Lewis will be able to cope well with the league and can make a big impact during his time here and kick-on from there."