The relevant passage in the Bible is easy enough to find: it is Ecclesiates 4:12 (see Bible Hub):
And if one prevail against him, two shall withstand him; and a threefold cord is not quickly broken. (King James Bible)
Finding the (possibly) corresponding passage in the Epic of Gilgamesh took a bit more effort.
Apparently, there is a close verbal parallel in Maureen Gallery Kovacs's translation of the Epic of Gilgamesh (Stanford University Press, 1989),
which can be found on Dust off the Bible and Academy of Ancient Texts.
(Since Kovacs's translation was published just over 30 years ago, I assume it is still copyrighted and that the owners of those websites cannot legally reproduce the text without the translator's permission or a licence from Stanford University Press, but I am unable to check whether these conditions were met.)
Kovacs's translation of Tablet IV contains the following passage:
'A slippery path is not feared by two people who help each
'Twice three times...
'A three-ply rope cannot be cut.'
'The mighty lioness cubs can roll him over."'
Kovacs's translation of Tablet V contains the following passage:
'A slippery path is not feared by two people who help each other.
'Twice three times...
'A three-ply rope cannot be cut.
'The mighty lion--two cubs can roll him over."'
This seems to confirm Wikipedia's claim, but the repetition of the same text on two different tablets looks odd; moreover, the elisions suggest that the tablets are very damaged here, so the translator may have made extrapolations to fill in gaps in the original text. For this reason, I also checked three other, more recent, translations.
Andrew George's translation (The Epic of Gilgamesh: The Babylonian Epic Poem and Other Texts in Akkadian and Sumerian. Translated with an introduction by Andrew George. London: Penguin, 1999. Reprinted with minor revisions in 2003) does not contain a corresponding passage in Tablet IV and presents the following passage from Tablet V:
[Two] garments, however, ......,
even a glacis-slope two [climbing can conquer.]
a three-ply rope [is not easily broken]
[Even] a mighty lion two cubs [can overcome.]
(The italics inside the square brackets are from George's translation; they indicate "restorations" that are not certain or simply conjectural. Regular text between square brackets is less conjectural because based on parallel passages that can be found elsewhere.)
Benjamin R. Foster's translation (The Epic of Gilgamesh. Translated and edited by Benjamin R. Foster. Second Edition. Norton, 2019) similarly does not contain a corresponding passage in Tablet IV and presents the following passage from Tablet V:
Though one be weak, two [together are strong].
If one cannot scale a slippery slope, two [can do it together].
A three-strand rope [is stronger when doubled],
Two cubs are [stronger] than a mighty lion.
(Foster does not use italics between square brackets to indicate a higher level of uncertainty about the content of the source text.)
Stefan Maul's translation (Das Gilgamesch-Epos. Neu Übersetzt und kommentiert von Stefan M. Maul. München: C. H. Beck, 2005. Sixth edition, 2014) does not contain anything that corresponds with the above passages. Presumably, the German assyriologist found the Akkadian text too fragmentary to translate.
What can be seen from George's and Foster's translations is that
- the statement about the "three-strand rope" is based on a conjectural restoration,
- both restorations differ from Kovacs's translation, and
- Kovacs reused a passage from Tablet V to fill a gap in Tablet IV.
My conclusion: to the extent that the Epic of Gilgamesh and Ecclesiastes 4:12 have a proverb in common, this appears to be based on a conjectural translation of a damaged passage in the Epic that translators formulated (whether consciously or not) to echo a passage from the Old Testament.