Gallio is redirecting console output to its Execution Log. So I needed to set the threshold of the ConsoleAppender to the level I want for the Gallio Execution Log.
<log4net>
<appender name="Console" type="log4net.Appender.ConsoleAppender">
<threshold value="ERROR"/>
<layout type="log4net.Layout.PatternLayout">
<!-- Pattern to output the caller's file name and line number -->
<conversionPattern value="%5level [%thread] (%file:%line) - %message%newline" />
</layout>
</appender>
<appender name="File" type="log4net.Appender.FileAppender">
<threshold value="DEBUG"/>
<file value="test.log" />
<appendToFile value="false" />
<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%5level [%thread] (%file:%line) - %message%newline" />
</layout>
</appender>
<root>
<appender-ref ref="Console" />
<appender-ref ref="File" />
</root>
</log4net>