As a native speaker of both languages, Cantonese /gw/ like in 過gwo3 and Mandarin /gu/ like in 过guo4 sounds the same, but I've checked that the Cantonese one is [kʷɔː] while the Mandarin one is [kwo], is there really a difference?
In an extremely narrow transcription:
- [kʷ] is a single sound, a type of [k] with rounded lips. The lips are rounded at the same time as the occlusion starts, and they're unrounded as soon as the occlusion ends (plosive burst).
- [kw] is a sequence of two sounds. First you get a [k], with unrounded lips; you'll get the occlusion and the burst. Then you get the [w], where the lips are rounded and the tongue gets close to the velum without touching it, as typical for approximants.
In practice... well. Speech does not segment so neatly. More often than not, you'll hear speakers using [kʷ], [kʷw] and [kw] interchangeably, sometimes even for the same word in different utterances. So when you're transcribing a language, there's a lot of room to transcribe all those sounds as [kʷ] or [kw].
Often the decision between one or another will be personal, and up to the author. Sometimes however you'll get people transcribing it as [kʷ] because, for the sake of phonotactics, it behaves like a single unit; while [kw] would behave as two units.
Sorry if this does not directly answer your question, as I don't speak either language. But hope that it helps anyway.