The 22-year-old was included in his 26-man squad for the upcoming tournament which kicks off next week.
After being left out of the Nations League fixtures, the midfielder played well with the Under-21s and then improved his form under new Blues manager Graham Potter.
Speaking at St George’s Park, Southgate says there are plenty of reasons to select Gallagher: “We were really pleased with how he responded at being out of our squad.
“He went into the Under-21s and performed really well. His mentality was excellent. It is a good example that if you approach it in that way that there’s always a route back with us.
“He has managed to achieve that straight away.
Adding: “He is fantastic at pressing the ball. I think there are going to be moments in these games where we are going to need different attributes.
“There are any number of players in this squad with different attributes to utilise at those moments. We feel he could be that sort of a player. He is not as experienced with us as some of the others. But he has an impact in games.
“He has got a goal threat. He, when you talk about midfielders, do they score goals, stop goals and create goals. He does a bit of all of that actually.“