According to flynn999 in this Wordorigins thread:
If you’re just saying someone is ginger-haired or ginger, factually, without any intended insult both g’s are pronounced soft (jin-jer). If you are being insulting it can be pronounced with a hard g and no soft g sound in the middle (ging-er, like singer), but by no means always or even the majority of the time, and its quite new. (I’ve taken to calling my tortoiseshell and white cat Ging Ging (like Sing Sing)). Its almost always more of a joke, though obviously some people can use it to be seriously nasty or bitchy
The learned Dr. Techie says “it sounds rather improbable,” and aldiboronti (a Brit) says “I’ve never heard a hard ‘g’ pronunciation in my life for any sense of the word.” As always, I turn to my readership for enlightenment: are any of you familiar with this alleged ginger-rhymes-with-singer pronunciation?