Voiceless: Pinyin h is standardly the voiceless velar fricative [x], although it is often written [χ] for some reason — Chinese IPA developed its own transcriptional traditions, for instance the use of [ɒ] where [ɑ] might be more usual, in the mid-twentieth century. However, there's no systematic contrast between [x] and [χ] in standard Mandarin, and systematic contrast is the important thing. Besides that, for many speakers, the aspiration of stops is also expressed with the velar fricative rather than pure aspiration: tāng: [tˣɐŋ], kuí: [kˣu̯ɘi], etc.
Voiced: In standard Cantonese, no. And in the major representatives of Yuè ("Cantonese") dialects, as described in Zhān Bóhuì 詹伯慧's Guǎngdōng Yuè fāngyán gàiyào 广东粤方言概要, voiced velar fricative [ɣ] is not attested.
There are some cases of [ɣ] in regional languages. Conservative Wú dialects, for instance, voice /x/ to [x͡ɣɦ] in low-register tones. Hokkien final /-k/ may take on voicing and approximant quality before the noun-suffix á: /sái ha̍k-á/ 'outhouse': [sai₅₅ ha₃₅ ɣa₅₂], and so on, although it is also often heard as a fully voiced stop [sai₅₅ ha₃₅ ga₅₂]. [ɣ] is reconstructed for the medieval initial xiámǔ 匣母 in most environments by most hands, too. But no dialect groups have anything like the degree of standardization we encounter in Mandarin.
