Commented by: Semyon Romanov (sas9mba)

I'm missing a file build_file.cpp
if you start the line of file generation:
C:\firebird-B3_0_Release\temp\x64\release\firebird\\gpre -n -m C:\firebird-B3_0_Release\src\msgs\build_file.epp C:\firebird-B3_0_Release\gen\preprocessing.cpp -b C:/firebird-B3_0_Release/gen/dbs/
Unable to complete network request to host "xnet://Global\FIREBIRD".
(E) C:\firebird-B3_0_Release\src\msgs\build_file.epp:47: Couldn't access database DB = 'C:/firebird-B3_0_Release/gen/dbs/msg.fdb'
1 error, no warnings

I'm also stuck building Firebird (currently 3.0.7.33374-0) with MinGW-w64 because I'm stuck on the .msg file generation and the sources don't include build_file.cpp

Unable to complete network request to host "xnet://Global\FIREBIRD".

That's FAR not ideal diag. gpre tries different ways to access DB but gives message from last one, which was netwiork one. Embedded access should work here but somewhy it did not work for you. You should solve that problem with your build to proceed.

What about MinGW build - it was not supported for many years, and if you have successfully reached message file generation point that's already great! What about build_file.cpp - build_file.epp should be preprocessed by gpre to get that file.