Sabayon Linux: GUI doesn't start (Unit keybase.gui.service not found)

When attempting to `run_keybase` on Sabayon Linux (Gentoo-based), I get the following output:

```Unmounting /run/user/1000/keybase/kbfs...
Unmounting and shutting down kbfsfuse...
Shutting down keybase service...
Starting via systemd...
Failed to restart keybase.gui.service: Unit keybase.gui.service not found.
Failed to launch GUI. Pass -g to prevent startup if on a machine without a graphical display.
run_keybase: Success!
```

Here's the output of `keybase status`:

```Username: halian
Logged in: yes

Device:
name: Stillstorm-Sabayon
ID: 86f3354be301c540df1d4c76d5fc9518
status: active

Session:
is valid: yes
keys: unlocked

Key status:
stream: not cached
secret: stored
dev sig: cached
dev enc: cached
paper sig: not cached
paper enc: not cached
prompt: show
tsec: not cached

KBFS:
status: running
version: 1.0.2
installed: 1.0.2
log: /home/halian/.cache/keybase/keybase.kbfs.log
mount: /run/user/1000/keybase/kbfs

Service:
status: running
version: 5.1.1
log: /home/halian/.cache/keybase/keybase.service.log
eklog: /home/halian/.cache/keybase/keybase.ek.log

Updater:
log: /home/halian/.cache/keybase/keybase.updater.log

Platform Information:
OS: linux
OS vers: Linux 4.20.0-sabayon x86_64-"Sabayon Linux amd64 20.03"
Runtime: go1.12.17
Arch: amd64

Client:
version: 5.1.1

Desktop app:
status: not running
version:
log: /home/halian/.cache/keybase/Keybase.app.log

Config path: /home/halian/.config/keybase/config.json
Default user: halian
Other users: halian
Configured accounts:
halian (current, logged in)
Known DeviceEKs:
deviceEphemeralKey-halian-1-1.ek
LocalDbStats:
Compactions
Level | Tables | Size(MB) | Time(sec) | Read(MB) | Write(MB)
-------+------------+---------------+---------------+---------------+---------------
1 | 13 | 91.93593 | 0.42045 | 95.32524 | 93.30474
2 | 128 | 663.99391 | 0.51270 | 72.26206 | 71.00473
-------+------------+---------------+---------------+---------------+---------------
Total | 141 | 755.92984 | 0.93316 | 167.58729 | 164.30947

levelDbCleaner{cacheSize: 45915, lastRun: 2020-04-19 19:40:32.205008546 -0400 EDT m=-3239.983229463, lastKey: [], running: false}
DbCleanerConfig{MaxSize: 2.1GB, HaltSize: 1.6GB, CleanInterval: 1h0m0s, CacheCapacity: 100000, MinCacheSize: 10000, SleepInterval: 50ms}

LocalChatDbStats:
Compactions
Level | Tables | Size(MB) | Time(sec) | Read(MB) | Write(MB)
-------+------------+---------------+---------------+---------------+---------------
0 | 2 | 7.37305 | 0.06841 | 0.00000 | 7.37305
1 | 9 | 90.78957 | 0.33547 | 59.47697 | 50.51146
-------+------------+---------------+---------------+---------------+---------------
Total | 11 | 98.16263 | 0.40387 | 59.47697 | 57.88452

levelDbCleaner{cacheSize: 212, lastRun: 2020-04-19 19:40:32.206661381 -0400 EDT m=-3239.981576619, lastKey: [], running: false}
DbCleanerConfig{MaxSize: 2.1GB, HaltSize: 1.6GB, CleanInterval: 1h0m0s, CacheCapacity: 100000, MinCacheSize: 10000, SleepInterval: 50ms}

LocalBlockCacheDbStats:

LocalSyncCacheDbStats:

CacheDirSizeInfo:
/home/halian/.cache/keybase: 404MB, (0 files)

command-line client:
keybase status [cid: 4, pid: 10978, version: 5.1.1]

KBFS:
/usr/bin/kbfsfuse -debug -log-to-file [cid: 2, pid: 10289, version: 1.0.2]
Notification subscriptions: Session, Favorites, Paperkeys, Keyfamily, Service, Kbfsrequest, Reachability, Team, Chatkbfsedits
```

I have also run `keybase log send` with log ID `6f2c2ad75fe9de590f623f1c`.

Reproduce the failure by running `run_keybase` on Sabayon Linux and inspect the systemd handling of `keybase.gui.service`. Use the reported `keybase status` output and log ID `6f2c2ad75fe9de590f623f1c` as diagnostics. Done means the GUI starts without the missing-unit error on the affected setup.

Written by the indexing model from the issue text.