Aguero went down under a challenge from Dejan Lovren during the first half of Sunday's Premier League fixture at Anfield, but the referee waved play on after deeming that Lovren had not fouled the Argentine.
However, Gallagher has suggested that Aguero was penalised for a desire to keep hold of the ball in the penalty area having quickly got back to his feet.
Gallagher told Sky Sports News: "I thought that it was a penalty. A lot of my colleagues don't, but it has split people's opinions.
"I don't think that he gets the ball but with Aguero popping up so quickly, I wonder if that gives the impression to the referee that Lovren got the ball and Aguero thought that he got the ball."
The game on Merseyside ended in a goalless draw.