The reader sees more of Gale Hawthorne inMockingjaythanThe Hunger GamesorCatching Fire.  Gale becomes more mercenary and determined in Mockingjay, because District 13 gives him the latitude and ability to contribute to the war against Panem.  From the very first book in the series, Gale has always been vehemently against the Capitol.  The changes the reader witnesses in Gale in Mockingjayresult from him not just talking about fighting the Capitol, but being able to actively do something about it.  

Gale's role dramatically shifts in the final book.  He originally was the caretaker and provider, not just for his family, but for Katniss' as well, especially during the Hunger Games.  When the Capitol fire-bombs District 12, Gale's priorities shift from survival mode to attack mode.  The destruction of his home district is the defining moment for him, because it forces Gale out of his role as the provider.  At District 13, everything is provided for, so Gale has no need to hunt to help his family survive; this new found freedom gives him the opportunity to help Beetee in the lab, devising cunning snares. 

Toward the end of the novel, Gale acknowledges how his changing roles from caretaker to soldier widened the rift between himself and Katniss: 

"That was the one thing I had going for me.  Taking care of your family" (367). 

Gale's role of soldier hardens him, forcing him to think analytically, even coldly about how to end the war the most quickly.  His idea about the double-exploding bombs possibly become the ones that kill Prim, and Katniss struggles to forgive him.  To District 13, however, Gale proves himself as a strategist, and at the end of the series, finds himself a "fancy job" in District 2 (385).