Tyrelle Harris, 20, fatally knifed the father in the chest at West Cross Shopping Centre on April 26 last year after a chance meeting.
Harris, of Helvellyn Way, West Bromwich, will have to spend at least 20 behind bars after he was convicted by jurors in March. He denied murder but pleaded guilty to possession of a bladed article.
Before sentencing him at Wolverhampton Crown Court on Friday, judge Michael Chambers QC said: "This was a deliberate stabbing of an unarmed man in a public place after you were carrying a knife.
"This was tragically yet another case in which those who carry knives reach for the knife at a moment of conflict and tension."
Mr Gallagher, who had not long qualified as a personal trainer, had travelled to the shopping centre with friends to buy food from F1 pizza, arriving at 4.30pm in a Seat Leon.
Not long after, Harris also arrived with a group of friends in a Hyundai and a confrontation took place; firstly between one of Harris' associates and Mr Gallagher, and then the defendant and Mr Gallagher.
Mobile phone footage, which was widely shared on social media, captured the moment Mr Gallagher was stabbed in the chest by Harris before he collapsed onto the floor seconds later.
Harris fled the scene and discarded the knife – which he had pleaded guilty to carrying – down a drain in a nearby estate, while Mr Gallagher was pronounced dead at 5.21pm, despite the best efforts of paramedics.
Mr Chambers continued: "It is accepted this was not a planned or gang-related incident but the main aggravating factor is you had a knife on you which you took to the scene.
"I have already indicated that I have found, as a fact to the criminal standard, that you didn't simply find the flick knife in the vehicle but you already had it.
"Shortly before 4.30pm you arrived on the car park in a Hyundai containing a group of young men. Already parked nearby was a Seat containing another group of young men including the deceased Karl Gallagher.
"To what extent these two groups already knew each other was unclear but it is clear from the CCTV that some greeted each other. But, as I already said, this was not a pre-arranged meeting or a form of gang conflict.
Watch the chance confrontation before the stabbing:
"Eventually, all of you were out of the vehicles. There came a point when there was a confrontation between some of the group. There appears to be a coming together of Karl Gallagher and Bennett.
"But then you moved to the back of the vehicle and Karl Gallagher approached you. As I have already indicated, there was some degree of provocation.
"He moved towards you, you moved moved backwards. He pushed towards you where you then took a flick knife. It is quite apparent from the way you flicked your wrist that you were accustomed to using such an item.
"You then moved towards him and he moved backwards and you stabbed him towards the chest. On the verdict of the jury, this was completely unjustified.
"He didn't have a weapon or gun. On the verdict of the jury, and I am satisfied, you didn't believe he had a gun."
For count one of murder, Mr Chambers said Harris will serve "custody for life, with the appropriate minimum term as one of 20 years" before a parole board can consider his release.
For possession of a knife, Harris was ordered to serve nine months, with that term to run concurrently alongside his murder sentence.