[Xrystal]

You have

EVENT_EXPERIENCE_GAIN (integer value, integer reason)
EVENT_EXPERIENCE_GAIN_DISCOVERY (string areaName, integer value)
EVENT_EXPERIENCE_UPDATE (string unitTag, integer currentExp, integer maxExp, integer reason)

The Update one occurs when someone levels in your vicinity.

The calling function you are using right but the function being used isn't being utilised properly.

local function UpdateXP(eventID,value,reason)
     CHAT_SYSTEM:AddMessage(string.format("You have gained %d experience.",value))
end
 
local function UpdateXPDiscovery(eventID,areaName,value)
    CHAT_SYSTEM:AddMessage(string.format("You have gained %d experience through discovering %s",value,areaName))
end
 
EVENT_MANAGER:RegisterForEvent("Mahalia", EVENT_EXPERIENCE_GAIN, UpdateXP)
EVENT_MANAGER:RegisterForEvent("Mahalia", EVENT_EXPERIENCE_GAIN_DISCOVERY, UpdateXPDiscovery)

As you can see the eventID is needed in the function you use to process the event. Some events are still to have their parameters discovered but they always have the eventID as their first parameter.

I also notice that you had it as GAINED rather than GAIN. Quite understandable.