Frankly speaking, 今行くがひつよう。is not correct and doesn't make sense. It's not necessary to translate 'need' into 'ひつよう', We don't really say ひつよう in our daily conversation. 今いかないと、is more likely to be spoken in Japanese. It's quite casual and is good to speak among your friends. :)

if you learn japanese in english way,you will be confused. they are totally different.you don't need to fill it in. you just need to remember a new language system.