Hi Luis,
Complementary Basic sensor fusion algorithm just uses Accel+Gyro
https://librepilot.atlassian.net/wiki/display/LPDOC/INS13+-+GPS+NavigationSo it will work for basic just flying. For
any GPS features you need
INS31 sensor fusion algorithm and
correctly working and calibrated
ALL sensors (accel,gyro,baro,mag,gps). In case just one sensor gives bad samples you won't be able to fly or sometimes even arm(safety check). You must have nice three curves stable without oscillations in Attitude states in Scopes with INS31, stable model in PFD and never ever Mag or GPS should be different from Green (no jumps to orange or worst red). Too many people just try and fail instead of fixing the problem and checking first that really all sensors are giving good data.
Most problems people have is with Magnetometer, which should be mounted on a mast (some GPS units include magnetometer because of that). This keeps that sensor away from all electronics. Also every GPS may have Mag mounted in different orientation so that should also be set correctly in GCS.
For good results I recommend 14-16cm mast.
Other problem people have is default altitude hold PIDs, this should be easy to test and tune in Altitude Hold tab by using Complementary Basic and Altitude Hold thrust mode. Coper should not oscillate/sky rocket but should maintain it's altitude. This PIDs are different from stabilization PIDs which of course should also be tunned to match your build for good stability and maneuverability (default ones are just a starting point that is determined such that it should work on most builds and allow you to get in the air).