Can you take the Grit rogue talent if your character has firearm proficiency from a source other than the Firearm Training feat?
Grit (Ex): A rogue with this talent gains the Amateur Gunslinger feat and one grit feat of her choice. She must fulfill the prerequisites of the grit feat in order to choose it. A rogue must have the firearm training talent before choosing this one.
RAW, no. They must have the firearm training talent.
So, in a society game, do not expect that to work.
Feel free to discuss with your game master if this is RAI, and what she or he might allow.
Yep, by the book you're forced to take the firearm training talent even if you're already proficient in all firearms. You can always try to convince your DM to wave the actual requirement and instead require proficiency in all firearms instead.