Someone may be able to assist you if you provide significantly more information about the configuration you've done, the messages you're receiving, and the diagnosis that you've already attempted.
Since you have an internal mail server and it probably already receives mail from gmail through other means, you may first want to talk with the people that maintain the internal mail server. They are likely to be the most familiar with the steps needed to allow gmail to be delivered to the internal mail server. If they are not already allowing delivery of gmail to the internal mail server, then they can probably explain to you why they are not allowing it and what you must do to request that delivery be allowed.