Has Gabriel Angelos worked with the Eldar?

Indeed, Gabriel Angelos has been cooperating with Eldars on more than one occasion:

Gabriel seems to have a mysterious bond with the Aeldari of the Biel-Tan Craftworld, particularly with Farseer Macha. Despite the condemnation and even threats from his fellow commanders, the Inquisition, the Adepta Sororitas and even from his own friend Isador Akios, Gabriel was stubborn in his attempts to assist the Aeldariand Macha on Tartarus. Surprisingly, the Harlequins of Arcadia refer to him as "Gabriel of the Hidden Heart," and see him as a symbol of hope for the Aeldari even though he is human.

HOWEVER, this information comes from books written by C.S Goto, who is generally despised by fandom, because often he demonstrated lack of basic knowledge about the 40k universe. Case in point - the Biel-Tan are the most xenophobic and racist of the Craftworld Eldars, so it is highly doubtful that they would cooperate with a mere human.

In chronological order:

1. Angelos fights with Eldars on planet Tartarus: Eldars are trying to prevent the escape of a demon imprisoned on this world. Gabriel attempts to kill the demon, but instead he just breaks his containment. [Dawn of War I]
2. On Rahe's Paradise, Eldars keep killing Blood Raven recruits with psychic potentials to prevent destruction of Eldari artifact keeping Necrons dormant. Gabriel attempts to negotiate the truce, but the destruction of a Spirit Pool of the Dark Reaper Aspect managed to awake the Necrons [Goto]
3. Lorn V/Arcadia: Angelos is contacted by Farseer Taldeer and with a small squad goes through webway to Aeldari World of Law, known to the Imperium as Arcadia, where with the help of a troupe of Harlequins he finds a legendary Eldar sword Blade of Vaul. Gabriel gives this blade to Farseer Macha, who sacrifices herself to contain the Necron threat [Goto]
4. On Acheron, Gabriel Angelos, now the Chapter Master of Blood Ravens is again fighting against Eldari threat (Including Farseer Macha, who somehow survives her "sacrifice"), but in the end they team up against demon infestation.[DoW III]

The general rule in Wh40k universe is: Everything published is canon, but not everything is true. Take your pick.

On the other hand, the Blood Magpies Ravens reputation for "obtaining" relics might be well deserved if you look at the various artifacts that they have access to (in Dawn of War II they have access to relics from other chapters, even one that belonged to bloody Custodes!). But this can be explained in other way than stealing: as a fleet based chapter with support from the Empire, they might have to rely on scavenging the battlefields - something that the real ravens actually do.

One is true: they are indeed hoarding relics, many of which they they should not have as it belongs to other chapters - please look at the official Blood Ravens armory:

• Axe of the Iron Snake (From the Iron Snakes chapter)
• Azrieel's second - sword from Dark Angels (a very secretive chapter itself)
• Bedlam Staff of Sinda Myrr (Alpha Legion)
• Doom of apostasy (a Dark Templar blade)
• Dorn's retribution (Iron Fist weapon)
• Fang of Fenris (Space Wolves)
• Holy Bolter of the Ancient (a revered relic from Ultramarines)

The list is long...