Regexp hate(Awk Sed)...! Any ideas?

Code:

G:     
sentence_to_join

OK I can get this to work by

Code:

sed '/G:\s*$/ {
N
s#G:\s*\n#G: #
}'

This works but I don't want.

Code:

G: 12.34.56 more stuff

So I think I can either do this with a not next line begins numerical in the sed i.e !^[0-9] ... But how?

Or my other thought was to search after for G:\s[0-9] but how do I then turn that into a newline in between the pattern? to end up with

Code:

G:
12.34.56 more stuff