If you mean the difference between gene flow and gene migration than the answer is none as they are synonyms.
The tell changes if you mean animal migration/gene flow.
Migrations are periodical movement (usually seasonal but not necessarily) along precise and often repeated trails; the trigger for the migration may be local climate local availability of food the season of the year or for mating reasons. To be counted as a true migration and not just a local dispersal the movement of the animals should be an annual or seasonal occurrence or a major habitat change as part of their life.
Gene flow is the transfer of alleles or genes from one population to another.
A migration does not implicates a gene flow which usually occur trough a local dispersal (immigration of an animal from an A population to a B population).
