Gul'dan would destroy Illidan. I can understand the allure for Illidan and why people are drawn to him, but he's not Gul'dan.
Illidan takes orders from his brother, both fled/captured by Maiev, and hid from Kil'Jaeden in outland. Grew in power considerably upon obtaining the skull of Gul'dan, and couldn't engage Tichondrius prior to obtaining the skull.
https://www.youtube.com/watch?v=2arN-vDwnNA
A lot of people only remember the engagement at TFT. Illidan certainly grew more powerful when they engaged at TFT, mostly losing because he was over confident. Keep in mind that Arthas was in a weakened state, and the LK gave him the last of the power he could give as his state was waning. Outside of that, you see that Arthas could hold his own against Illidan prior to the skull, which is sort of humorous.
While he looks ferocious, nothing really stands out as impressive on the physical side of his combat, especially for somebody who has been alive for over ten thousand years. While adept at magic, most of the magic we witness him doing is closing/opening portals, which while impressive doesn't really aid one necessarily in combat.
Gul'dan controls demons, opens portals to other realms seemingly easily and is both manipulative and extremely cunning. In a bout with Illidan, assuming they were both prepared, it would be Gul'dan telling Illidan that he wasn't prepared. Look how easily he bound Grom, granted he was likely prepared for it. The only way I see Gul'dan losing to Illidan is if the plot demands it, and Gul'dan was caught off guard by Illidans nature being unpredictable (i.e Gul'dan lets Illidan out of his cage assuming he would aid Gul'dan, only for him to jump at him immediately kinda thing).