One of the 3Z Ginpachi Sensei omake has a funny one, in which Tomokazu Sugita tells about how he got a call from his mother after the Stand Inn arc was aired, telling him he really should work on his singing.
Episode 150 includes a frame-by-frame parody of a scene in Neon Genesis Evangelion. Hasegawa takes the place of Gendou Ikari, both of whom are voiced by Fumihiko Tachiki. He also, sometimes does some mannerisms as Gendou.
In episode 122, before he was revealed, the Eyecatch has his silhouette shaped like Kenpachi
Taken to the extreme in fight between Elizabeth (Tōru Furuya) and Dark Vader (voiced by Shuichi Ikeda), to resemble the fight between Amuro Ray and Char Aznable in Mobile Suit Gundam. The staff even made Elizabeth pilot the RX-78 Gundam.
Hijitaka says LET'S PARTY! in in his introduction as hosts in the Takamagahara host club. The difference is, instead of a horse, Hijikata uses Kondo as his Harley Gorillason!
Upon Gedomaru's arrival in episode 249, Gintoki yells Saber!
Cash Cow Franchise: For Sunrise mostly, leading to people not even believe when they say they are 'stopping' with the animation series once at a while. Gintama is one of the best sold anime series on DVDs in its origin country and merchandise come and go because the popularity of it.
On a sidenote, among all the major characters, Tomokazu Sugita is the only one that never voice act a Gundam pilot.
No Export for You: Okay, right, Sentai Filmworks has the first season of the series licensed, the series can STILL be watched through Crunchyroll not only in the States, but many countries with the service as well, and the Benizakura-hen movie was also licensed and released with an English Dub... BUT, the other tons of episodes of Gintama can't be reached in a home release form, and Viz Media cancelled the publishing of the series in Volume 23 for both low sales and because translation difficulties (killing in part also the possible license of similar comedy (and comedy action) series that rely mostly in the wordly dialogue and Japanese jokes and heavy words such as Sket Dance, Beelzebub and one Jump's recent hit, The Disaster of PSI Kusuo Saiki).
Played straight with Sugita (again) and Kazuhiko Inoue (Oboro) since they eventually fought against each other, again, in Jojo's Bizarre Adventure Part 2 as Joseph Joestar and Kars. It is straightforward, however in the JoJo's Bizarre Adventure: All Star Battle game applying some what ifs and strange cases, and different cast from Part 3's anime adaptation and subsequent ones if they get animated. Ayako Kawasumi, Gedoumaru, did Erina Pendleton (later Joestar), who is Joseph's grandmother, too.
LITERALLY. Both as an in-universe example and in real life, Gintoki and the Leukocyte King.
Episode 26 (or 227, depending on your point of view) has it when characters from Sket Dance meet the Yorozuya gang: Tomokazu Sugita as both Sakata Gintoki and Switch. It's both checked in their correspondent crossover anime episodes, EVERYONE (with the exception of Kagura literally talked with themselves. Yes, even Ryōko Shiraishi who got casted as the monkey (Jugem-monkey).
Throw It In: The initial title of Yorozuya Gin-san was rejected by the editor, and another one was needed. Expecting that his other ideas would be shot down too, the author decided to play a little joke and submitted a random title his dad had made up: Gintama.
Trolling Creator: Sorachi is known as the greatest troll on the other side of Japan. There's even a special idiom for him, "Gorilla is playing with reader's crotch again."
Hideaki Sorachi once wanted to do the series in base of the Shinsengumi. In fact, Shinpachi and Gintoki were once conceived as police units of the group. Hasegawa would eventually lead them (technically in the series he once worked with the Shinsengumi at the pretty start of the serialization, but his original role in the first ideas of the series was replaced by Kondo) and a girl who ended up being separate into Okita, which hairstyle was used so much later for Otsuu (and technically a reused idea for his genderbent identity in an arc that happened 9 years later the serialization started), and Kagura (she had an umbrella as a weapon, which became the defacto weapon of use for her and the Yato clan).
The title of the manga was originally going to be Yorozuya Gin, reflecting the nature of Gintoki's "work", but the editors of the magazine didn't like it, so it ended up being "Gintama".
The Be Forever Yorozuya movie was originally stated for a release in Spring 2013, just after the anime new season (reruns of some selected episodes, plus the airing of new such as the Kintama arc, and the Courtesan of the Nation arc) ended. However, as proved in the anime previews, the script seems to have suffered many changes in the process, making it move until a more proper release in Summer. The original ideas were discarded, and it makes it wonder what would haven't worked instead of the final result: a Bad Future story involving the dissapearance of Gintoki, and a sickness that has been spreaded towards all Kabukicho in his absence.