Expy: Van is designed to look like Spike Siegel, complete with with badass tuxedo and hat. However, he also has a lot of similarities with Domon Kasshu: A Jerk with a Heart of Gold who is hell bent on revenge, thinks his allies are just nuisance, whose main partner (Wendy being the counterpart for Rain Mikamura) is mostly on the passive side (not fighting), and also has a GenkiAction Girl who falls for him but eventually they don't hook up (Priscilla being the counterpart of Allenby Beardsley).
The luck-obsessed guy from the first episode seems to be based on Simon Phoenix.
Shout-Out: El Dorado references a lot of Super Robot anime, but in particular it showcases several direct references to GaoGaiGar, for example using the "G-Stone" sound during it's transformation poses and having a visual shout out to King K-Der's Mega Fusion.
Watch their debut episode. Seriously, the enemy pilot that said that the Robot is outdated ? Voiced by Nobuyuki Hiyama. El Dora V is a GaoGaiGarExpy. Yeah...
And a bit more on El Dora V: Ground vehicle forming legs looks very similar to Mammoth from Dancouga, the way arms are formed is similar to Voltes V and Combattler V, and cockpit arrangement with all pilots moved from their individual cockpits to one large control room is usually haw various Sentai combiners work.
In terms of the El Dora pilots, their dead female member is named Chizuru Stephens, who shares the same first name as the token female pilot in "Combattler V", Chizuru Nanbara.
El Dora is one big walking shout-out. Nero and Jose at one point reminisce on fighting the Zaurian empire, which sounds a bit close to Getter Robo's Dinosaur Empire.
At one point, Van finished an opponent with the V no Giri, Voltes V's finishing move.
This combined with the disregard for his little sister ends up coming up in Super Robot Wars K as Sbinn (now part of the time) has some angry words for him.
The first episode ("Tuxedo Blowing In the Wind") clearly borrows elements from The Road Warrior. Compare Lucky's speeches to the speeches of The Humongous, for instance, or note the way a bus is used to block the entrance to the mall.
Word of God on this anime has some odd things to say. In an Anime Insider interview, the director insisted that GUN×SWORD is actually NOT a western. Apparently, the creators preferred to think of it as a mecha anime rather than a Space Western.