[Leigh Butler, Steven Hillage]
Mat seems to think so. In WH, Noal and Mat watch the Ebou Dar gholam escape: "The creature stuck its hands into a hole left by a missing brick...Hands followed arms, and then the gholam's head went into the hole...The gholam's chest slithered through, its legs, and it was gone. Through an opening maybe the size of Mat's two hands." Noal comments that he's never seen anything like that before. "'I have,' Mat said hollowly. 'In Shadar Logoth.' Sometimes bits of his own memory he thought lost floated up out of nowhere, and that one had just surfaced, watching the gholam" [WH: 16, An Unexpected Encounter, 355].
Here's the bit Mat is thinking of: "As Mordeth dove through the air, he stretched out and thinned, like a tendril of smoke. As thin as a finger he struck a crack in the wall tiles and vanished into it" [TEOTW: 19, Shadow's Waiting, 240].
However, despite Mat's certainty and the similarity of the two descriptions, it doesn't seem possible that Mordeth is a gholam. There are lots of reasons why it's unlikely: If Mordeth is a gholam, how did he eat? It's not like people waltzed into SL on a regular basis, ripe for "harvesting". Gholam are physical beings - why didn't Mordeth have a shadow? And for that matter, how could a corporeal thing like a gholam have merged with Fain? Possession by a spirit is one thing, but how could a gholam-body merge with Fain's body? And why would it want to? Why hasn't the Ebou Dar gholam tried Mordeth's swelling-to-huge-proportions illusion that he did in TEOTW (to try and trap the boys in the treasure room) [Nevin Aiken]? If Mordeth was a gholam why couldn't he leave SL? Moiraine states, also in TEOTW, that no denizen of SL, including Mordeth, can cross her ward lines, but can't gholam melt OP flows with ease?
Additionally, the descriptions above are similar but not identical. Mordeth's evokes a smoky or misty image, while the gholam in WH (and in all other descriptions we've had), gives a far more liquid-like impression.
All these problems make the idea unlikely, but the reason why it's pretty much impossible is this: we know gholam were created by Aginor as a tool of the Dark One. If we posit that Mordeth is a gholam, then how do we reconcile this with the statement that SL evil did not come from the Dark One, but from the suspicion and hate of the people of Aridhol, who had been poisoned by Mordeth?
Mordeth, and Aridhol itself, are just as opposed to the Dark One as the forces of Light are, just in a bad way. Thus supposing that Mordeth is a gholam-- a weapon of the Dark One-- makes no sense.