Outstanding this term on-loan with Charlton Athletic, Gallagher struck a wonder goal for England U21 in their Euros qualifying defeat of Albania last week.
While on duty with England U18s last summer, routine checks discovered Gallagher had a heart condition.
Minor surgery quickly resolved the issue but cost the youngster the time to prepare for the upcoming campaign.
"It definitely drives me on because it's a bit of a scare, isn't it?" Gallagher said.
"It makes you appreciate it more when you're out there on the pitch playing.
"You have to give 100 per cent because every single one of us is lucky to be here. Ever since it happened, after the surgery, I've just wanted to prove myself.
"I've come back fitter, stronger. I've set myself small targets and I feel that is the better way to do it because I didn't expect to be starting on Friday night and I did and then did well. I can set more targets."