How to set ErrorLog location with SessionCreator.exe?

I would not modify the environment variables as other application might be using it.

You can use parameters /ExportLog and /ExportSummary for SessionCreator.exe. It will look something like this,

GITLAB_USER_LOGIN: "domain\TestUser1"
CI_PROJECT_DIR: "MyProject"

SessionCreator.exe RunTest /UserName:${GITLAB_USER_LOGIN} /UseActiveSession /ProjectPath: "C:\TestCompleteBuild\automation\Automate.pjs" /ExportLog: "${CI_PROJECT_DIR}\ExportLog\TestCompleteTestReport.html"

Note, you might have to use double backslashes (\\) to escape backslashes in Windows-style paths.