At the end, I have changed my strategy.
Instead of creating the SPEC file with program gem2rpm, I just prepare it manually to copy the .gem file into /tmp/, and then run "logstash-plugin install" against it.
It seems to be working fine.
Only annoying thing is that the plugin gets installed under vendor/local_gems/ instead of vendor/bundle/jruby/2.5.0/ with the rest of plugins. I guess I need to set some environment variable or similar to fix that.