[Fizzlemizz]

local addonName = ...
local AutoSelectGossip = {
    [48598] = true,
    [109275] = true,
};
     
local function IsAutoSelectOption(gossipOptionID)
    return gossipOptionID and AutoSelectGossip[gossipOptionID] == true
end
addon.IsAutoSelectOption = IsAutoSelectOption;

You haven't defined addon. My guess is it is suppsed to be your addon's private table (NameSpace) so:

local addonName, addon = ...
local AutoSelectGossip = {
    [48598] = true,
    [109275] = true,
};
     
local function IsAutoSelectOption(gossipOptionID)
    return gossipOptionID and AutoSelectGossip[gossipOptionID] == true
end
addon.IsAutoSelectOption = IsAutoSelectOption;