The 22-year-old started the first three matches of the season under new coach Nathan Brown but has not featured in Super League since and recently played on a dual registration for Rochdale in the Northern Rail Cup.
Gaskell trained with his new Reds team-mates on Thursday morning and will make his debut against Huddersfield at the Salford City Stadium on Friday.
Howarth, 23, was an ever-present at Salford under Phil Veivers last year but, after missing the start of the season through injury, has made just one substitute appearance under interim boss Alan Hunte.
The former Wigan and Wakefield player is set to go straight into the Saints team for Friday's derby against the Warriors at the DW Stadium, instantly providing cover for injured hooker James Roby.
Brown said: "At this stage of his career Lee needs to be playing more regularly and, with chances limited at present, it made sense for him to go out on loan.
"We're looking forward to watching him develop and I'm sure he will do a good job for Salford.
"Stuart has shown in his time at Salford that he is a capable and experienced player and will prove more than adequate cover for a multitude of positions.
"He will be put straight into the Good Friday squad and we are confident he will do well."