Should I use PromptPurchase and PlayerOwnsAsset to see if they own a gamepass and buy one?

No. PlayerOwnsAsset no longer works with game passes, due to the new game pass update. Game passes are no longer assets, in fact. They are their own category. You should be using MarketplaceService.UserOwnsGamePassAsync.

-- Script in ServerScriptService
-- example 
local MarketplaceService = game:GetService("MarketplaceService")
local id = 0000000000 --id

game:GetService("Players").PlayerAdded:Connect(function(player)
    if MarketplaceService:UserOwnsGamePassAsync(player.UserId, id) then
        -- argument is the UserId, not the player!
    else
        MarketplaceService:PromptGamePassPurchase(player, id)
    end
end)