it's where you go to get tested for sexually transmitted diseases
it is but in general they are yes but they also offer other services so i wouldn't stigmatize it with STD's soley, I'm not embarrassed to say I've been to them here in UK past few years for general sexual health check i ended up getting my Hep. B vaccine re-boosted even though I'm no longer employed by NHS but as they detected my vaccine was low i got a booster jab :) cant grumble i also had the HIV/Aids test which you can refuse to have carried out if you wish ... but it's nice to know sometimes that you are good in terms of sexual health :)
but in general for the OP they are more often these days referred to as Sexual Health Clinics than GUM which is short for genito-urinary medicine.