You have to show your attempts at answering the problem to get help, this is a forum policy.
So far you have not gave enough information to make teh question reasonably precise, way too many unknowns about the sample composition.
Start with checking which of the possible compounds (or compound families) present can be separated by each of the techniques. Perhaps there are compounds which can't be separated at all neither with hplc or gc? Then neither of these techniques will work for the mixture.