The three GRP issues
Despite its excellent suitability as a boat-building material, GRP has three important characteristics. Firstly, to a greater or lesser extent, all resins used in boatbuilding are permeable, which means they will allow water to enter; secondly, almost all GRP contains voids and hollow areas inside the layers of material; and thirdly, there are almost certainly going to be residual chemicals in the material, some of which are hygroscopic, which means they absorb moisture in the same way as salt.

How do you avoid osmosis?
In light of the three GRP issues outlined above, there is an argument that all GRP boats are susceptible. There is also some evidence to suggest that boats kept in warmer waters are more prone to the problem and that saltwater is better for GRP boats than fresh. In the UK, therefore, a boat kept permanently afloat on a mooring in the upper reaches of a river is more susceptible than one wintered in the boatyard and launched into salt water each season.
Above all however, it seems that the quality of the original build plays an important part. Poorly constructed hulls, with voids and dry-spots, thin porous gel coats, and sloppy processes can encourage osmosis in later years. If you’re buying second-hand, check out the original builder’s reputation. And if you’re buying new, get reassurance that care in the building process is more than just a token gesture.

The inevitable result
Given these three qualities, it is inevitable that, over time, small quantities of water will seep through the gel coat and perhaps even find their way out through the other side into the bilges. If this were always the case, there wouldn’t be much of a problem. En-route however, water molecules may meet and dissolve residual hygroscopic chemicals, creating highly concentrated acidic chemical solutions which begin to fill the voids, attracting more water in turn, which accelerates the breakdown of the GRP.
The acidic solution building up within the layers has a higher molecular weight than the water that originally entered, so it cannot leave the GRP in the same way as it came in. Eventually, blisters will appear, perhaps weeping small quantities of acidic vinegary-smelling liquid. It is an accelerating process that adds weight to the vessel and breaks down the material strength of the hull.  
Once you have it, you cannot eradicate it by simply reversing the process. Drying out the hull only stops the process until the boat is re-launched. The cure has to include a removal of the hygroscopic chemicals, otherwise the process will start up all over again.