Я сделал это:
require 'wikipedia'
require 'ap' #awesome_print
page = Wikipedia.find("cats")
ap page.categories
И получил это ([ n ] предоставляется awesome_print):
[
[0] "Category:Accuracy disputes from December 2011",
[1] "Category:Accuracy disputes from November 2011",
[2] "Category:All articles with dead external links",
[3] "Category:All articles with specifically marked weasel-worded phrases",
[4] "Category:All articles with unsourced statements",
[5] "Category:Animals described in 1758",
[6] "Category:Article Feedback 5",
[7] "Category:Articles containing Ancient Greek language text",
[8] "Category:Articles that may be too long from February 2012",
[9] "Category:Articles with 'species' microformats"
]