Gu means 'to (a place or time)'. So it's a to you can use when 'going to the shops'. But not when listening to a fiddle. True it has a lot of idiomatic uses, but pretty sure when you're listening to things you use ri
There are exceptions such as "rinn" but I'm not sure if this is idiosyncratic or standard. It seems to depend on the consonants & vowels (broad v. narrow) in the following word. The pronunciation of ri is as you say.