The critical distinction is with regards to what level of certainty is the requirement. In a Judicial Evidence Gathering, the intention is to acquire enough information that will pass legal muster, and the 'beyond a reasonable doubt' for criminal cases, and a 'perponderance of evidence' in the rest of the cases.
Intelligence gathering is about dealing with 'risk assessment' - and therefore deals with the probabilities that both the information is 'sound and valid' so as to be within the probabilities that the risk is assessed at the appropriate level.
The problem is that the popular image of 'para military forces' swinging into action to go and 'get information' is not quite the right approach to this, since it confuses the 'military intelligence gathering' aspect with the rest of the community. There is also the problem that with the end of the cold war, there are not quite as many places where 'they are', whom we can cleanly say to ourselves, 'lie beyond the reach of the law', where such operations can be conducted in 'peace time', or at least in the context of the so called "cold war".
The problem of failed nation states remains the open ambiguity. It is not always clear that there is any 'law' available that can be appealed to as an arbitrating constraint about the means and methods by which information is acquired. So the process of 'intelligence gathering' by other than lawful means should be a limiting factor in the modern process of 'intelligence gathering'.
Hum... this is no easier to describe here than my first shot at it.