[votan]

In addition to Ayantir:
GAMEPAD_TOOLTIPS seems to be the official global name.
:AddLine accepts a dynamic list of styles. A "style" is a table which contains style-values. /zgoo ZO_TOOLTIP_STYLES to see which values are supported. The style-value defined first wins => The priority goes from left to right.

-- ....
local mystyle = { fontSize = 34, fontColorField = GAMEPAD_TOOLTIP_COLOR_GENERAL_COLOR_1, }
local function AddInfo_Gamepad(tooltip, itemLink)
    if itemLink then
        tooltip:AddLine("Whatever text you want to add", mystyle, tooltip:GetStyle("bodySection"))
    end
end
 
-- ...
 
local function TooltipHook_Gamepad(tooltipControl, method, linkFunc)
    local origMethod = tooltipControl[method]
 
    tooltipControl[method] = function(self, ...)
        origMethod(self, ...)
        AddInfo_Gamepad(self, linkFunc(...))
    end
end
 
-- This helper function is just there in case the position of the item-link will change
local function ReturnItemLink(itemLink)
    return itemLink
end
 
local function HookBagTips()
-- ...
 
    TooltipHook_Gamepad(GAMEPAD_TOOLTIPS:GetTooltip(GAMEPAD_LEFT_TOOLTIP), "LayoutItem", ReturnItemLink)
    TooltipHook_Gamepad(GAMEPAD_TOOLTIPS:GetTooltip(GAMEPAD_RIGHT_TOOLTIP), "LayoutItem", ReturnItemLink)
    TooltipHook_Gamepad(GAMEPAD_TOOLTIPS:GetTooltip(GAMEPAD_MOVABLE_TOOLTIP), "LayoutItem", ReturnItemLink)
end
--...

The first parameter of :LayoutItem is an item-link. You have to adapt the TooltipHook_Gamepad to your needs. It's just an example to start with.