I am huge, longtime Gundam fan in terms of the franchise and I don't care what anybody says: I bloody LOVE Mobile Fighter G Gundam! ^__^ Now granted, the old 80's/90's martial arts shonen genre was pretty much my cup of tea, so yeah, there's a bit of a "targeted-audience fanboy" bias but appreciation for G is lot deeper than that..
For starters, like H.D.Inuyasha just mentioned, is was the first "Alternate-Universe" Gundam series and honestly, rightfully so, IMO. I personally felt that after watching Victory Gundam, the series in terms of its tone, style, and appeal was personally becoming rather stale (and prior to that, F91 wasn't exactly a box-office smash either). It seems like for whatever reason (BANDAI's influence?) they decided to break the mold and change their target audience and what was the in-thing back in the early-mid 90's? Martial arts shonen! How is this significant? It expands your target audience which is crucial for a marketing franchise and plants the perverbial "seed" when your audience is a kid and as they grow older then they can expose themselves to the older classic series which they may not have given a crap about otherwise.. The Gundam franchise continued to expand their target audiences beyond the sci-fi/mech heads with Wing, X, SEED, SEED Destiny and keep up to date with the changing times and interests..
The other significance is in regards to Kunio Okawara's mech designs which introduced "thematic-type" Gundams to the franchise like Dragon Gundam, Maxter, Heaven's Sword, Master, etc. Gundam's that had a theme or were a special-type of MS that excelled in a specific area. Prior to that, the titular Gundams were pretty much Military-standardized Gun/beam saber/maybe a gimmick type. G Gundam paved the way for unique specialty types that led to Gundam designs like Deathscythe Hell, Altron, Raider, Epyon, Wing Zero, Buster, Heavyarms, Leopard, etc. So design-wise, it was also a trend setter for the franchise..
Now granted, it didn't follow the traditional themes as much but as I mentioned before, it broke the monotonous mold and was a catalyst for taking the franchise to a new level.
