I realize that this is an old thread but it never hurts to pass some info along to others who might have a similar problem.
Multiwii uses two modes that are very similar but still different. You have Angle (also known as level) mode, and Horizon. The two modes seem identical until you go crazy with the sticks. In Angle mode, it will never do a flip. Just shouldn't work. Now in Horizon, if you have your pitch/roll rate set up with higher figures than zero, you can fly calm and easy like in level mode, or if you decide you want to get crazy you can do flips and stuff. All in Horizon mode. Another flight mode is called Acro. In that mode you are only flying using a gyro with no other positional sensors active. That is the one where you can do anything but relax. As far as getting your GPS to work, do you have everything properly set up in your config.h file? In Arduino config.h you must enable the correct serial port (#2 on MW Pro) define it's baud speed, initialize it, go back and comment initialize, then uncomment NMEA then upload again. You must also set your magnetic declination to your flying location. All of that is done in the GPS section of your config.h file. When you have everything set up there, you will need to calibrate your mag. I calibrate my mag before each GPS flight or mission. Then you need to be sure you have at least a 5 satellite GPS lock before trying to fly. I see you have the MTK GPS module. It isn't one of the best ones out there. Ublox works way better and you don't need to initialize it before getting it to work correctly. You need to have your RX wires hooked up to your TX and your TX hooked up to RX. You need to make sure that you have your //#define USE_MSP_WP looking like this #define USE_MSP_WP by removing the two slashes. Also, GPS and waypoint missions are not really fully supported in MW 2.3. You should upgrade to 2.4 where all of the code in other sections of the MW sketch have been upgraded to functioning status for GPS.
I see on your screenshot that you have default RC expo and RC Rates set. RC Expo (RC exponential) reduces stick sensitivity towards the centre of the sticks range of travel. More RC Expo makes for smoother flying with sticks centered without reducing maximum travel when the sticks are at the edges. Yours at 65 is a pretty decent setting.
RC rate is just a simple scale used to tell the aircraft how much it should respond to a given stick input. More RC rate makes for increased sensitivity to stick inputs. Put this up for acrobatic flying. Too much makes smooth flying almost impossible. Yours being at 90, it could stand to be set back to around 35. Thr. Mid (throttle Mid point) just lets you set the mid point on your throttle stick. Yours being at 50% might be right if your craft hovers at mid throttle. If you need to move your throttle past half way to hover or less than half way to hover, you would adjust your mid to reflect where you actually hover. If it takes 75 percent of your throttle sticks throw to hover, then set your mid to 75. Some fine tuning of that number may still be needed to get the craft to hover at half stick.
Thr. Expo (throttle Exponential) makes the throttle more sensitive at the edges and less sensitive towards the center of throttle stick travel. More throttle exponential makes holding a smooth stable hover easier. I have mine set at 35 as it makes for a much more tame throttle response. I see in your Nav setting menu that you have takeover baro enabled. This will allow your GPS to automatically use baro when needed so you don't really need to switch it on in aux. Also, should you get return to home working, the settings have your craft rising to a height of 15 meters before trying to return. You can set that to whatever you feel safe with. I see you have both GPS filters on. You should probably only go with forward prediction filter instead of both. I see that your Pos Hold and Hold Rate PIDs are still at default. Try changing them some. My Pos Hold P is at .40 and my I is at .50. My Pos Hold Rate P is set at 4.0, my I at .20 and my D is .050. My position hold works good with those settings. Your numbers will probably be different but you could try mine. One other thing. GPS on these quads doesn't always work even when things are set up correctly. They are sensitive to fluctuations in the Earth's magnetic field, solar flares, cloudy skies and the list goes on and on. But on a good day mine works great and I can fly waypoint missions.
One more thing. If you update your firmware to 2.4 but still use the 2.3 Nav version of the Win GUI, you will need to go into your Multiwii Arduino Multiwii.h sketch and change the version number to 2.3 (it will be 2.4) then upload that sketch to your flight controller.
