As a general rule in Scottish Gaelic, the initial consonant cluster gn- in Scottish Gaelic is pronounced as though gr-. The change from n to r is frequent in Scottish Gaelic and most generally applies to words beginning with cn-, gn- and to some extent tn-. This also varies quite a lot. However across some dialects n becomes r to much greater extents, this is especially present in the Gaelic dialects of the northern mainland. 

Sn is generally pronounced not with an r sound, although a special kind of mutation occurs in for example “sneachda” - after a preposition and in the definite form, where for example “anns an t-sneachda” - “in the snow” is frequently pronounced as though “auns a dreachga”.

The initial consonant cluster gn- is most commonly pronounced gr-, although this does not occur everywhere, and for example gnè - kind, whilst generally being pronounced as grè, is pronounced as gnè in a part of northwest Lewis according to what I read from (1). This word is also pronounced as gne by one speaker at the top of North Uist, as well as being pronounced gnè at the bottom of Sleat on Skye, and at Diabaig on the north of Loch Torridon. All other parts of Scotland seem to use the pronunciation grè or a variant therefore, except for St Kilda of course which pronounced the word as glè or grè. 

The word gnothach - business, is almost universally pronounced with an initial gr-, the only exceptions being northwest Lewis, where the form gnothaich is found, and again northernmost North Uist, where the form gnòch is found, both with an initial gn-. 

Words beginning with sn- are generally pronounced sn- in most dialects, although the word snàmh - swim does have the unusual pronunciation of swàmh in the Cairngorms area according to (1), which seems to coincide with a tendency in this area towards certain sounds (but not c and g) developing a w, for example “nature” is nwàdur in some of these areas. Similarly snàithlean - “thread” becomes snwàain in this area according to (1), and snaim - “knott” becomes swà̰im, whilst the form snwaim is found at the bottom of Harris, as are the forms snwà and snwàileæn for snàithlean. 

Whilst I will not draw any conclusions, the area from southern Harris and onto the top of North Uist seems to represent an area where some of these initial clusters may have been pronounced differently from an early point. Does this in some way perhaps relate to an influence or influences specifically around the Sound Of Harris, and could that in turn relate to prehistoric landscapes in that area. 

The area around the Cairngorms shows such a different phonology to elsewhere in Scotland that I presume at least one of the ancient linguistic influences there could have been very different. It could relate to one of the languages spoken by the Picts in the east or it could be more specifically connected to the indigenous peoples around the Cairngorms, or more likely to both perhaps. This is not to say that their language was Brythonic nor that the similar forms on Harris are Pictish, and again, this part of Harris also has words beginning with lw-. 

As we might expect, words that begin with tn- in Gaelic like tnùth - “envy” also occasionally have the n sound preserved. And perhaps not surprisingly according to (1) the n sound is preserved in parts of Northern Lewis and at the top of North Uist, although this word is not commonly represented in the survey and there are no other examples with tn- given. 

Note that the Gaelic words in this article given in dialectal spelling are spelled based upon the phonetic information given in (1), the Survey of the Gaelic dialects of Scotland, edited by Cathair Ó Dochartaigh. I hope that this article was an interesting read.