What is the difference between migration and gene flow?

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).