I found a lot of information concerning the Hxaro relationship (eg: Wiessner 1982), but very little concerning !gu!na relationship. Marshall talks about it but doesn't compare it with the Hxaro.
Marcus & Flannery, K. (2012) talk about it:
Hxaro was not the only system used by the !Kung to create networks of partners. There were also networks of !gu!na, “namesakes,” built on the premise that names were magic. [...] Arriving at a distant camp, a visitor needed only to give his shared name to be welcomed by the family of his !gu!na.
Lorna Marshall (1957) talks specifically about the !gu!na. She provides the only example I found about what it concretely means :
Gao, the brother of /U, went on an errand for us to Khadum.[...] He had never been to Khadum before, and the /Kung Bushmen who lived there at once called him ju dole [(strangers)]. He was in haste to say that he had heard that the father of one of the people at Khadum had the same name as his father and that another had a brother named Gao. 'Oh,' said the Khadum people, in effect, 'so you are Gao's !gu!na ', and they took him to their fire and gave him a present of edible gum. [...] The /Kung who live in this region are not ju dole to each other. The name-relationship makes them feel they are one people.
I suppose !gu!na is a very light version of Xharo since the partners don't know each other, and thus don't know if the other will give back. What is the rule of !gu!na partnership (eg. what should they do, for how long, in what condition...)?