From the sounds of it, either your IRIS settings or the gimbal settings are not quite right.
IRIS/Pixhawk T-2D Settings
First thing to try is to make sure you have the correct settings on your T-2D Gimbal. To do this you will need to connect it to your PC via the USB Dongle included with your kit. To start, please donload the setup software and drivers here
Inside the zip file you will see the drivers folder, within that you can install the drivers. If you are using windows 8, please see this post on windows 8 prolific drivers to get this to work properly.
Once the drivers are installed connect everything up to your PC and open the ZYX-BMGC-EN_V1.5.exe file to open the config tool. Here select the correct COM port (usually highest number) and you should be able to read your gimbal settings.
The best thing to do is to download the settings file for IRIS/Pixhawk and write it to the board just to make sure you have the correct settings for your Pixhawk/IRIS. You can download the settings parameter file here - 3DR_Tarot.bgsc file.
Check IRIS/Pixhawk Settings
To check your IRIS/Pixhawk has been configured correctly for the T-2D gimbal, connect to via mission planner and go to the Initial Setup > Optional Hardware > Camera Gimbal screen set the parameters according to the screenshot below:
Hope that helps.