I don't know exactly what you mean by "Error message in sourcetype", but in props.conf, you should have something like this:
TIME_FORMAT=%d/%m/%Y %H:%M GMT%z
The regular expression is not wanted or needed to describe the time in props.conf
You really don't want to use both %Z and %z in the timestamp. I would just go with the UTC offset, as clearly "GMT" is not the actual timezone, but the base for the offset.
You may have other settings in the stanza within props.conf that affect timestamp parsing as well. If this doesn't solve your problem, can you show us the entire stanza in props.conf?