To put it simply it just means the girl licks your butt hole.
It's also called DD which is an acronym for Dragon Drilling.
If a girl puts a lot of effort into cleaning your bum in the shower, it usually means you are gonna get DD
Guan service includes DD and a few other things, like catbath, BBBJ, sometimes CIM, massage etc.
It's a professional type of service where the customer basically relaxes and the girl pampers and pleasures him and serves him like he is a king.
It's named after the Chinese city of Dongguan which was the Chinese centre of prostitution for many years - mainly for locals, not really for Westerners.
The authorities clamped down on it so it's not as big as it was but it's still going on on a smaller scale and underground.
Many Chinese WLs with experience in Dongguan or China in general will provide this service,
You have probably already had this type of service dozens of times without even knowing it.
For example nearly all the Chinese girls at Ginza and 278 Club provide this service, any maybe some in the budget shops too.