Propylene Glycol is a diol alcohol (a type of organic alcohol), meaning it contains two hydroxyl groups in it's chemical structure, and it has an asymmetrical carbon atom. Because of this, the liquid is hygroscopic, meaning that it has a natural ability to attract water molecules from the surrounding environment. Because PG is also fully miscible (term that means the property of liquids to mix in all proportions, forming a homogeneous solution), it will eventually release moisture into the air when the relative humidity drops below that of the PG solution's proportion of water.
Purchased PG solutions are mixed at approximately 60-40 distilled water/PG. Because there is already water in the solution, the PG has attracted and mixed with it. In other words, PG solution is water AND PG, so it sets a baseline for the PG to balance at around 70%. This is why if you add PG solution to an already well humidified environment (70+%), it takes a little more time to balance out than if you added it to an environment nearer 70% RH.
The gel (crystals in Xikar humidifiers) that you wet with PG solution is simply a silica that absorbs and extrudes water based compounds, hence organic alcohols like PG that are water based. When combined, the gel will absorb the PG solution, which will in turn attract water from the surrounding environment, or eventually give it up to humidify the air.
The silica beads are similar to the gel in function (chemically different though). They are also a desiccant, which means they will also absorb humidity from the air. Depending on the chemical bonding order of the atoms, various forms of silica have various absorption levels, rates, and capacities.
The one myth to debunk. Plan silica gel or beads do NOT "extrude" moisture. Rather when the ambient humidity drops below a point, the beads simply dry out as the humidity is wicked back out of them. The bonding order and surface area of the silica used determines how fast they lose humidity (or gain it).
Because of PG's natural willingness to "give off" moisture, silica gel and PG solution will help bring humidity UP faster, while silica beads will take longer to lose moisture. Dry beads will always trump dry gel to remove excess moisture, and that I don't have a real good answer to.
Anyway, hope that helps some!