Had some issues following some of the posted guides online when trying to convert the Lazarus IDE to GTK2. The first portion worked, whereby you compile the LCL to GTK2 first, but the second step, which was to compile the IDE itself kept failing. I eventually got it working by compiling it from the command line. I did a: make clean all make LCL_PLATFORM=gtk2 and it worked. Please note to do all of he above as superuser / root.