Building AOSP with Gapps built in-line:

1. Sync AOSP

2. Edit manifest to include:
<remote name="opengapps" fetch="https://github.com/opengapps/"  />

<project path="vendor/google/build" name="aosp_build" revision="master" remote="opengapps" />
<project path="vendor/opengapps/sources/all" name="all" clone-depth="1" revision="master" remote="opengapps" />

<!-- arm64 depends on arm -->
<project path="vendor/opengapps/sources/arm" name="arm" clone-depth="1" revision="master" remote="opengapps" />
<project path="vendor/opengapps/sources/arm64" name="arm64" clone-depth="1" revision="master" remote="opengapps" />

<project path="vendor/opengapps/sources/x86" name="x86" clone-depth="1" revision="master" remote="opengapps" />
<project path="vendor/opengapps/sources/x86_64" name="x86_64" clone-depth="1" revision="master" remote="opengapps" />

3. Then re-sync

4. Edit device.mk
https://github.com/patch-Adams/android_device_moto_shamu/commit/e51dd0107dc3eb27b6181f39d1fc08fa05e7e400
Note: you may have to play with the gapps package size that is in line 23

5. Edit aosp_"device name".mk
Comment out this line:
PRODUCT_RESTRICT_VENDOR_FILES := true

6. I don't like stock recovery to build.  When you flash you either have to remove it from the /out dir or you'll lose TWRP
https://github.com/patch-Adams/android_device_moto_shamu/commit/1971779634d7a985461335df2bda2a39fa53b104

Then build!  AOSP rarely breaks so you get a lot fewer build breaks than with other roms :)