The global 50 m mosaics available to download from http://www.eorc.jaxa.jp/ALOS/en/palsar_fnf/fnf_index.htm are mapped binary files with an ENVI header. You should just be able to read them using GDAL with the ENVI driver (I've not had any problems doing this).
You need to apply the following equation to calibrate HH and HV data to gamma0
gamma0 = 10*log10(DN^2) - 83.0
It is not possible to use these mosaic data for interferometry as the phase information is lost during the processing (you need the Level 1.1 single look complex data data for interferometry). The processing is described in:
M. Shimada and T. Otaki, "Generating Continent-scale High-quality SAR Mosaic Datasets: Application to PALSAR Data for Global Monitoring," IEEE JSTARS Special Issue on Kyoto and Carbon Initiative, vol. 3, Issue 4, 2010, pp.637-656.