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 5 goals in three starts for United's under-23`s 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.
Gateshead manager and former Newcastle favourite Steve Watson 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."
He could go straight into the Gateshead side for today's trip to Chester City.