Bob the Concise Oxford gives "girn" as a spelling variation of "gurn" with the meaning "to pull a face".

In Scotland it has wider meaning than that - and it is more commonly used to mean "to complain peevishly or to whimper"

So for instance we might say someone is "aye girnin" meanin "always moaning"; or a baby who is always crying will be called "a girnie bairn"

"dinnae come greetin and girnin" for "don't come crying and moaning" etc.

It is one of those Scots words that is commonly used in Scottish Standard English too.

But yes I have seen the funny "gurnin" competitions in northern England too with photos of usually old men pulling faces. I am sure there are probably other uses too.