He actually sings, "I'll be seeing my gran and seeing my mum." In Britain, gran is a name for a grandmother (probably short for granny, which is short for grandma, which is short for grandmother). The plural, then, would be grans, but I don't think you'd hear it often. More likely, British folk would use a different word like grannies or grandmothers to talk about more than one gran.

http://www.lyricsmode.com/lyrics/t/tim_minchin/white_wine_in_the_sun.html

http://dictionary.reference.com/browse/gran

If you look at the lyrics to “White Wine in the Sun”, you’ll see that he actually sings,

I’ll be seeing my dad,
My brother and sisters, my gran and my mum.

In Britain, gran is a name for a grandmother (probably short for granny, which is short for grandma, which is short for grandmother). The plural, then, would be grans, but I don’t think you’d hear it often. More likely, British folk would use a different word like grannies or grandmothers to talk about more than one gran.