A319 First Officer reports both FMGC's dumping when NO STAR is selected on the MCDU resulting in an empty flight plan page and Map Not Available on both map displays. IMC prevails and crew elects to go around. Attempts to reset the FMGC's are unsuccessful and crew diverts in VMC.

Narrative

While flying base turn to final Runway 36R; PF while autopilot engaged; attempted to transition from SUDSY 2 STAR to approach VIA for Runway 36R. This was especially pertinent since ATC had vectored our aircraft 26 NM Southeast of the CLT airport. An approach VIA provides an auto load of the five fixes listed on the approach to Runway 36R (HEKAM; JENDA; LANSR; CIKDU; and HAYOU) instead of just the two that normally load (CIKDU and HAYOU). This is important for position awareness and also because CLT ATC often vectors you direct to; or in reference to; these extended fixes; especially when IMC. When inserting the Approach Via using the MCDU key 3L 'No Star' all data from both FMGC's dumped and we were presented with an empty 'Flight Plan' page 'Map not Avail' on both map displays. Shortly thereafter; GPS Position Lost; then GPS Primary. After using the 'Standby Nav' function to tune the ILS frequency 108.9; we noticed full scale deflection on both Glide Slope and Localizer Course. At this point; due to being completely IMC; thunderstorms moving in from the west; and triple parallel approaches being conducted by CLT Approach Control; we elected to break out and remain VMC toward the northeast. Shortly after trying to change the destination in the MCDU as a way of resetting it; we failed; and thus decided to divert under VMC. It is important to note that all performance; weight; and fuel load data also dumped. The 'direct to' key worked normally once GPS primary came back. FMGC's both reset upon shutdown at the gate. Possible software anomaly. The 'Approach VIA' key does not work as designed when loading an approach into the MCDU. (It stopped working after a software update sometime around Jan 2010). The existing method of forcing an Approach VIA via the 'No Star' key is an approved work around by Airbus Flight Training. Request the manufacturer rework the original 'Approach VIA' key to work as originally designed.