dnd 3.5e – What metric could be used to convert a mental stat between 3.5 D&D and GURPS?

When converting character back and forth between 3.5 D&D and GURPS, there are a few stats that are easier than others.

As an example, an easy stat such as Strength could be converted by using the weight load values using the metric of weight to find the equivalent values, or possibly by using the metric of damage dealt to find equivalent Strength values.

Non-physical stats are more troublesome as they do not always have numerical values that map well to each other.

What metric or method could be used to compare and convert a mental stat, such as INT from D&D and IQ from GURPS?