A GEL Script running a XOG will not fail unless there is an connection error or a problem with the setup of the XOG message. The only way an error will be logged is if you programatically test for error or warning messages from the result string and log a GEL error. In most cases in the field, people test for a SUCCESS result only.
That being said, if you are trying to move data from a child object to a parent, there are a number of ways to do it with a modern PPM instance. If the value is numerical or an Enum based lookup, consider adding Aggregated Calculated Attribute to the parent object. These offer a number of options to summarize numerical values to a parent object, without needing the process engine.
If is the a String-based lookup value, then you can have the process itself set the value, rather than using a GEL script.
Without seeing what you are trying to do with your GEL script, it is hard to diagnose what is causing these data copy failures and to suggest alternatives.