Indeed gcj was removed from gcc compiler suite for the release 7.1. The openjdk compiler should be used instead. I managed to build the module on opensuse by setting gcj = no in the Makefile, and set a proper JAVA_HOME on the command line. The module seems to initialize ok:
0(20759) DEBUG: <core> [core/sr_module.c:909]: init_mod(): app_java 0(20759) INFO: app_java [app_java_mod.c:131]: mod_init(): Initializing Java VM with options: -Djava.compiler=/usr/lib64/jvm/java/bin/javac -D/usr/lib64/jvm/java/jre/lib/:/home/henning/repositories/kamailio/src/modules/app_java/Kamailio.jar -verbose:gc,jni 0(20759) INFO: app_java [app_java_mod.c:150]: mod_init(): app_java: Java VM initialization OK 0(20759) ERROR: app_java [java_support.c:180]: handle_exception(): app_java: Exception: Exception in thread "main" java.lang.NoClassDefFoundError: Kamailio
Your installation method by trying to match the proper kamailio version of a downloaded version to a package version is not recommended and might produce strange errors.
I would suggest to compile the complete kamailio all together, you will find a tutorial e.g. in our wiki.