hi eng mohamed,the tutorials of gems already in his site https://www.2comu.com/support_download/download.html
if u face any problem,pls tell me,did u use it?,i hope i can use it in EBG simulation
asalamo alykom
In the same website (https://www.2comu.com/) you can find OpenGEMS, which is an open source free FDTD solver. Unfortunately it doen't have the plane wave simulation feature.