甲, read 갑 in Korean, is like many other Hanja in having several meanings.
Here, it most likely stands for the first of the ten 天干 (천간) or "heavenly stems", an ancient Chinese enumeration system. The ten "stems" are still commonly used in cultures that use Sinitic characters as ordinal number-equivalents in things like multiple choice questions (instead of a,b,c,d,e etc) and can be also used to enumerate sections in legal documents, subsections in dictionary entries (especially Hanja dictionaries) They are also used in Taiwan, at any rate, for student grades, and so I assume they were thus used in China as a whole at one time for that purpose.
If I've taken the right sense of the character, then in the album title, the 6 has a double function. It's PSY's sixth album, but it also contains 6 songs, and calling it 6甲 gives it the sense of "6 Grade A" or "6 First Class" songs. Or maybe "Six of the Best", though since that brings back painful memories of what we got as 11-year-olds from our draconian Latin master if we failed to recite all the inflected forms of the paradigmatic second declension adjective "bonus" without taking a breath, I personally dislike that phrase.
Adding to 조금만, 6甲 is also a vulgar expression meaning 'words' or 'behaviour'.
There is a series of letters starting with 甲, which are used for fortune-telling. Fortune tellers in traditional Korean culture jibber-jabber with these sixty-words(6甲) when telling people their future. And later the word 육갑 also means 'someone's cheap words or behaviour to convince people in a lousy manner'.
I believe this is what this singer intended. He wants to describe his music 'shallow words and vulgar dance' in a satirical and self-mocking way.
Also, I think this is his sixth album. So, 6甲(육갑) makes perfect sense.
That sort of pun would be both very Korean and very PSY, so it makes perfect sense.
I hadn't previously encountered 6甲 as a free-standing expression with this colloquial meaning before, but I wonder whether it isn't a contraction (on the pattern used, for example, to name major highways by the first hanja in the start and end location of the road) of the much more familiar term 六十甲子 or "sexagenary cycle", which refers to the 60 characters (starting with 甲 in the sense of the first of the 天干 or "heavenly stems" I referred to) in combination with the 12 地支 or "earthly branches" (the twelve "animals" of the Chinese zodiac), the first of which is 子 (read 자 in Korean), hence 甲子 to refer to the whole 60 by naming the first item in each of the two component series. (I won't claim to understand the arithmetic which derives only 60 combinations rather than 72 from 6 times 12 entities, and I've yet to see it convincingly explained).
Each of the 60 combinations of a "stem" with a "branch" stands for one of the 60 years of the Chinese cosmologial/atrological cycle, and someone who has lived through all 60 of these pairings has experienced the full cycle of existence, which is why the 60th birthday is such big occasion in all cultures where the influence of Chinese astrology is still felt. The stem+branch pairing of the year, month, day and hour of a person's birth (4 pairings, hence 8 characters in all) are that person's "Four Pillars" [四柱, 사주] (or sometimes "Eight Characters" [八字, 팔자]) that decide the individual's destiny in Chinese astrology.
That's why these 60 two-character combinations figure so largely in what kenjoluma refers to as the "jibber-jabber" of fortune tellers. So perhaps the original expression might be translated as "mumbo-jumbo", as used perjoratively in English of language which the listener thinks in nonsensical and suspects the speaker doesn't really understand either ("It's all just management mumbo-jumbo").
It's perhaps not totally irrelevant here (by my capacious standards of relevance at any rate) that fortune telling via scrutiny of people's "four pillars" (especially those of a couple planning to marry) is still very widely practised in Korea, even by people who seem otherwise more or less completely "Westernised" in their lifestyle and outlook. The recent TV drama "A Wife's Credentials" had a striking instance of this, where a highly educated and sophisticated woman whose marriage is in trouble consults a fortune teller (who has all his astrological data on a high-end Samsung laptop) who sternly tells her that the problem lies with her young daughter's name. By rashly choosing a name they liked rather than one that harmonized with the four pillars of the two parents, they have doomed their marriage to failure unless the little girl's name is changed forthwith, which the mother immediately decides to do. (It doesn't work.)