You should be able to add an email account if you open and run the email application. Which is a terrible application, it doesn’t support IMAP IDLE, which means that it has to poll your email server to get updates. I’d suggest k9mail for email. https://github.com/k9mail/k-9/releases
You aren’t going to get a “gapps” account, because there are no gapps. Google services are proprietary software, so they need to be licensed from google. Their proprietary nature makes them inconsistent with the objectives of AOSP and Linaro.
However, if you really need to install some element of interoperability with your google account, you have the FREE option of the microG project: https://microg.org/ – note that it requires a source code patch in order to enable itself to pretend to be google’s proprietary services, so patch, rebuild, install, setup. There will still be no play store, but there are alternatives to that as well. I like F-Droid myself; https://f-droid.org/en/ – which distributes open source software exclusively.
Now as far as the non-booting of AOSP master, it can be somewhat unstable. From one day to the next, it might not boot. I’d suggest starting with one of the “known good” manifests, which are basically somewhat tested snapshots of AOSP master. https://github.com/96boards/aosp-known-good-manifests
Now there is one more thing about AOSP master. In January, it was switched to default to kernel 4.14. There are some graphics mode regressions in 4.14 that I don’t think have been cleared up yet, so there is a slight potential for that to be messing you up. You can run a build with kernel 4.9 to test that. Note that you need to do a full “make clean” or otherwise wipe the out/ directory when switching kernel versions. To build with 4.9 just set the variable TARGET_KERNEL_USE=4.9 when building, so build comes down to;
TARGET_KERNEL_USE=4.9 m -j$(nproc)
When build is complete, go into the path
AOSP_ROOT/device/linaro/hikey/installer/hikey960/ and run