Thinking more about the sudden shift to third person in "he fell apart with his broken heart"... 'He' could be referring to Peeta, when he hit the force field and his heart stopped. It became obvious then that Katniss really cared for Peeta (Finnick himself said so), and maybe that's when blood drained from Gale's skin/face, became pale from the shock of this revelation.
Thinking more about the sudden shift to third person in "he fell apart with his broken heart"... 'He' could be referring to Peeta, when he hit the force field and his heart stopped. It became obvious then that Katniss really cared for Peeta (Finnick himself said so), and maybe that's when blood drained from Gale's skin/face, became pale from the shock of this revelation.