Obtaining 100,000 Gold will definitely take a while. Without level up bonuses and daily/weekly bonuses, you get around 50 gold per Free For All match (highly recommended as it is the fastest method.) It would take around 333.33 hours from 0. Don’t worry, you get 120 gold from account level ups, and certain character level ups (the first being 7, and alternate until 21(last color at 20) but once you unlock all stances/colors each character level is 120 Gold). Daily rewards give from 50 to 250 (EDIT 3/15/19: holidays give a 250 bonus), and daily missions give 135 (Win 1 match as X character or X character OR play 2 matches as one of those characters). (Another EDIT 3/15/19: Brawl Of The Week gives 300 for 2 wins.) In reality, it should take around 100-150 hours from 0, but you will probably obtain over 25,000 going for everything else. I hope I could help by providing useful information. Let me know if I can improve this guide. EDIT: According to SUPER FLY 1988, you do not need to have all of it at once. However, I would recommend levelling each character to around 13 or 15 for gold bonuses.
FINAL NOTE 4/14/19: I just finished this achievement, and it seems to decrease your gold earnings once you reach account level 50. I was at about 99,600 Gold when it drastically slowed down, my final game gave me 2 gold, so it might cap at that. This may vary by player since most will wait until they are really close to start grinding, but I was smooth sailing with about 600 Gold an hour depending on if I took breaks or got a lot of bonuses. I just wanted to add my experience in case you guys have the same happen.
9/13/19: Thanks to AbsolTheDarkOne for the information, it seems that daily challenges have been reworked to take longer and give 250 gold.