[Q] include gapps in local build topic

Before I start I want to emphasize: This is for my personal build only. If I ever plan on releasing a build I will remove gapps from my build.

Now after the disclosement. I am somewhat new to building AOSP and after flashing my build many times I am tired of flashing and than flashing gapps. I have the flashable gapps.zip file and am looking for a way to add the zip into the `fastboot flash system` process. Is there any way to *locally* add this zip into the build for personal use? (I have a nexus 6 so this is for android 5+)

Thanks in advance