I'm Scottish. When I see an r in the middle of, or at the end of, a word I pronounce it. The same goes for most Americans and some English people. I think it's therefore a confusing way to indicate a long vowel. Wouldn't it be clearer to say Gahna?
I agree too with whoever said Gh doesn't indicate that the following vowel is going to be long. Here's Wikipedia on the subject:
In English ⟨gh⟩ historically represented [x] (the voiceless velar fricative, as in the Scottish Gaelic word Loch), and still does in lough and certain other Hiberno-English words, especially proper nouns. In the dominant dialects of modern English, ⟨gh⟩ is almost always either silent or pronounced /f/ (see Ough). It is thought that before disappearing, the sound became partially or completely voiced to [ɣx] or [ɣ], which would explain the new spelling - Old English used a simple ⟨h⟩ - and the diphthongization of any preceding vowel.
It is also occasionally pronounced [É™], such as in Edinburgh.
When gh occurs at the beginning of a word in English, it is pronounced /É¡/ as in "ghost", "ghastly", "ghoul", "ghetto", "ghee" etc. In this context, it does not derive from a former /x/.