if I don'tถ้า means “if,” กู is a very informal and often vulgar way to say “I,” and ไม่ negates the following verb or adjective. Use only among very close friends or in heated, highly informal speech.
TH: ถ้ากูไม่ไปวันนี้ เราจะไม่ได้เจอกันอีกนะ
EN: If I don't go today, we won't see each other again.