In both cases it is spelled and pronounced as one word. Both '-es' [flat] and '-is' [sharp] are added.
Ges [G Flat] is pronounced like 'guess' in English (but with the 'u' not stressed too much]. Gis [G Sharp] in German sounds like 'kiss' with the 'k' spelled as soft as in "guess" …, like 'giss'.
Most musicians I know use Gsharp and Gflat because C, D, E, and G sound quite familiar :). In case one isn't sure, s_he uses the Ges for G Flat and so on.
I seem to have been informed wrongly on Geses e.a. and I am deeply sorry for havin spread that misinformation! I didn't delete my text below to keep up the context of the comments.
There is also a hardly outside classical music used variations like Geses [G Flat Flat = F] and Gisis [G Sharp Sharp = A]. This would be spelled like 'guesses' and 'gisses'. You might find this used among some classical musicians, but very rarely, as it makes no real sense.