хахаха/hahaha -usual laugh
хихихи - "supressed" sort of laugh, usually then you should not laugh. mouth is not wide open and "ха" sound becomes something like "хи" through smile. (note, "giggle" is translated to russian as "хихикание")
гагага/гыгыгы - sort of "vulgar", strong laugh. vocal cords are stressed and thin "х" hardens to something like "г".
so, "гггг" is supressed гыгыгы.