Because both of these techniques do the same exact thing. They basically separate the proteins based on size. But what's the major difference between these two methods? Well, in gel atrophies all of these proteins in the mixture are forced to move through the porous gel. But in the case of gel filtration chromatography, only the small proteins are actually forced to move through the beads that are composed of that porous gel. In gel filtration chromatography, the large proteins are not forced to move through the porous beads.