We're missing the actual file you want signed (gcp_installer.exe, right?). When you added it to the archive config:

https://crrev.com/943ff34f6e5fdb79934ef3e5e23cc1343965eba0/chrome/tools/build/win/FILES.cfg#513

I think it needs to be configured for:
'filegroup': ['default', 'symsrc'],

and not just 'symsrc' (similar to the chrome.exe config).

Correct, gcp_installer.exe is to be signed.  I didn't add it the config file because there is no need for pdbs.  I'll write a CL to update the config file.  Can I just use 'default' if we just need signing and don't need pdbs?

It should probably be configured like mini_installer.exe, so it gets archived directly (separate from any .zip file or anything):

https://chromium.googlesource.com/chromium/src/+/943ff34f6e5fdb79934ef3e5e23cc1343965eba0/chrome/tools/build/win/FILES.cfg#600

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/418dc7ae76bba60cc8253c4532051cc4ef1c3c80

commit 418dc7ae76bba60cc8253c4532051cc4ef1c3c80
Author: Roger Tawa <rogerta@chromium.org>
Date: Fri Nov 09 23:19:27 2018

Add gcp_installer.exe to archive config so that it can be signed.

Bug:  902890 
Change-Id: I57021c1d018a48869aecf404854566bbf0b5dabd
Reviewed-on: https://chromium-review.googlesource.com/c/1330080
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607021}
[modify] https://crrev.com/418dc7ae76bba60cc8253c4532051cc4ef1c3c80/chrome/tools/build/win/FILES.cfg

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/release/scripts/+/96505a80d72d1aa5c8ebfe7abfb9e7c1916836a4

commit 96505a80d72d1aa5c8ebfe7abfb9e7c1916836a4
Author: Michael Moss <mmoss@google.com>
Date: Mon Nov 12 21:05:46 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/release/scripts/+/4a9d8491e295da7f9203b608382096660f76b01a

commit 4a9d8491e295da7f9203b608382096660f76b01a
Author: Michael Moss <mmoss@google.com>
Date: Mon Nov 12 23:40:47 2018

We can't really test it until tonight's build, but all the pieces should be in place now. Feel free to reopen if you don't see signed packages tomorrow.