It's interesting to see where his opponents went after beating them. Here are the 16 opponents he knocked out in his title defenses and where they went afterwards (before these fights, he fought a bunch of tomatoes in Germany).
Starting from his first title fight:
Milton Nunez (Active) - He had one loss to his name and was only 22 when he got KOed by GGG. Has gone 11-13 since then.
Nilson Julio Tapia (Retired) - Fought once more and won by decision before retiring at 25.
Kassim Ouma (Active) - Took two and half years off after getting knocked out in the 10th round. Has fought only three times since losing to GGG, going 2-1 in those fights.
Lajuan Simon (Retired) - Took two years off before fighting J'Leon Love and losing by knockout. He has since retired.
Makoto Fuchigami (Active) - Has gone 3-4 since getting knocked out by GGG in 2012.
Grzegorz Proksa (Retired) - Went 1-2 before retiring at 29.
Gabriel Rosado (Active) - Has gone 1-3 since losing to GGG. Drew against Curtis Stevens in
"Big Knockout Boxing" in 2015.
Nohuhiro Ishida (Retired) - Actually moved all the way up to Heavyweight after losing to GGG. Went 3-2 against unknown fighters in Japan before hanging them up.
Matthew Macklin (Active) - No longer ranked in the top 10, but still active and performing fairly well. He has gone 6-1 since fighting GGG, though mostly against outmatched competition.
Curtis Stevens (Active) - Went 2-1 in 2014 after losing to GGG in 2013. Then, he fought Gabriel Rosado in an abortion of a BKB match in May 2015 that resulted in a majority draw. Will fight Brazilian middleweight prospect Patrick Teixeira in 2 weeks.
Osumanu Adama (Active?) - Went 2-0 against two Ghanaian tomato cans since his loss to GGG. Hasn't fought since April 2015. It is unknown if he will fight again.
Daniel Geale (Active) - Has gone 1-1 since getting KOed by GGG. He beat journeyman Jarrod Fletcher by unanimous decision before getting knocked out by Cotto.
Marco Antonio Rubio (Active) - Has fought only once since losing to GGG in October 2014, losing to Anthony Dirrell by unanimous decision
Martin Murray (Active) - Has gone 3-1 since getting KOed by GGG. His three wins were against unknown tomato cans while his loss was an uninspiring split decision loss against Arthur Abraham.
Willie Monroe Jr (Active?) - Has not fought since losing to GGG in May 2015.
David Lemieux (Active) - Fights Glen Tapia in 2 weeks
Dominic Wade (Active) - TBD
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The first 8 fighters on the list were either massively weakened or outright retired due to the beating they suffered at the hands of GGG. Adama and Monroe Jr. haven't fought in the last year. So, who's to say whether or not they come back? Active fighters like Macklin, Murray, and Geale have done well against journeyman but haven't held their own against any viable contenders. Stevens has slowed up dramatically as well. Wade doesn't appear to have much of a future in boxing and we'll see what happens with Lemieux.