Kinda late, but allow me to explain.
Yes, ごめんなさい means sorry. The quotation marks you're talking about are called "dakuten", and it's only used on a few of the hiragana and katakana characters.
They're used for the k-line, s-line, t-line and h-line. So that means:
k-line: かきくけこ (ka ki ku ke ko) becomes がぎぐげご (ga gi gu ge go)
s-line: さしすせそ (sa shi su se so) becomes ざじずぜぞ (za ji zu ze zo)
t-line: たちつてと (ta chi tsu te to) becomes だぢづでど (da ji zu de do)
h-line: はひふへほ (ha hi hu he ho) becomes ばびぶべぼ (ba bi bu be bo)
Similarly, there is also "handukaten" but it's only used on the h-line. In that case, はひふへほ just becomes ぱぴぷぺぽ (pa pi pu pe po)
All of this is just for the sake of creating a larger variety of consonant sounds in Japanese characters and they're used in the same way for katakana characters. In fact, some hiragana and katakana charts do have dakuten and handakuten characters as well.