do/a vs gu

[Eòs Buidhe]

When do you use gu rather than do/a? I realize do is used as "for" and gu as "until" in different contexts, but I'm more confused on when they are being used to mean "to", as in "I'm going to a ceilidh." Would both Tha sinn a' dol do cheilidh and Tha sinn a' dol gu ceilidh be correct? How about Fàilte gu Alba and Fàilte a dh'Alba?

[Níall Beag]

All distinction in language are fuzzy, so take nothing as concrete, but "gu" usually implies "direction" whereas "do" means "destination". Or at least that's the meaning for people who say "fàilte do...". The "fàilte gu..." people seem to be emphasising distance of travel.

For example "thoir dhomh e" is give it to me -- no implied distance. But "cuir litir thugam" is send it to me, over a distance.

[Seonaidh]

To elaborate slightly (but possibly too elaborate...), "gu" might be "towards" or "up to", whereas "do" might imply something beyond merely "up to", e.g. "into" etc. But it's fuzzy - and often either can be used - apart from in set verb thingies where a particular one implies a specific meaning the other one doesn't. One would say, for instance, "[bh]o àm gu àm" and not "[bh]o àm do dh'àm/ a dh'àm"