Magnetic fields play an important role in the ecosystem of galaxies. They are dynamically important and influence many physical processes such as star formation, gas dynamics, and cosmic ray acceleration and propagation. The most versatile observational method for galactic magnetic fields is radio polarimetry, in particular the recently developed analysis technique of rotation measure (RM) Synthesis. In principle, RM Synthesis allows disentanglement of different magnetized components along a line of sight, which is not possible using traditional methods (like synchrotron emission measurements). However, for this method to reach full potential, extremely broad frequency bandwidths are needed. The Square Kilometre Array (SKA) will be able to provide these, but none of the current radio telescopes. However, that barrier is overcome with the on-going Global Magneto-Ionic Medium Survey (GMIMS) project, which consists of 6 surveys: 3 broad frequency bands in 2 hemispheres. The GMIMS survey will for the first time be able to reveal magnetized structures that are invisible to current surveys. We propose to exploit the GMIMS data sets that are currently available to (1) study Galactic magnetism and its relation to the interstellar medium (ISM), and (2) investigate techniques to optimally extract magnetized components along the line of sight, using broad but not continuous bandwidths. Firstly, we will search for correlations of the detected magnetized structures with ISM components, allowing determination of their magnetic fields. Magnetized structures without ISM counterparts are likely to be part of the turbulent magneto-ionized medium, allowing us to derive turbulence parameters from their statistical properties. Secondly, the GMIMS survey provides us with the unique opportunity to compare the performance of various known spectro-polarimetric analysis techniques (RM Synthesis and alternatives to it), and develop our own technique, for optimal use of spectropolarimetric data sets in future radio telescopes like the SKA.