What does it mean when Katniss recalls the moment with Gale when he says " We are using the same rule book President Snow used when he Hijacked Peeta"

It bascially means that they have no compassion for the enemy. That even if it is morally wrong and a disgusting act towards humanity, it doesn't matter because the people they're killing aren't on their side. Gale turned into a monster in Mockingjay.

It means that they are not really doing anything wrong they are doing what the president did they are simply getting back

Just how snow had no mercy neither will gale

I cant really recall that part but i think its when they are rebelling the capitol or around then
cause thats when they did it to peeta ( i was so furious when they did that to Peeta they almost ruined Katniss and hims future!)