What is the purpose of the viscous gel in the Pacinian corpuscle?
Short answer
The adaptation process in Pacianian corpuscles (PCs) is mediated via the outer capsule. This capsule is an onion-like structure that quickly molds itself to pressure stimuli, thereby rapidly desensitizing the receptor. 

Background
The Pacinian corpuscles are rapidly adapting (RA) skin receptors, as opposed to the slowly adapting (SA) ones, such as the Merkel cells. RA cells become swiftly unresponsive (i.e., they adapt) when a pressure stimulus is applied, but faithfully transmit rapidly changing stimuli (such as vibrations).

Pacinian corpuscles are specialized vibration receptors. Their dendritic region is shaped as an onion-like structure with layers of stacked lamellae:


Specialized dendritic capsule of the Pacinian corpuscle. Source: Biologymad  

These lamellae act as high-pass filters that result in a sharp drop in sensitivity below 150 Hz (Johnson, 2001). If this capsule is dissected from the receptor and a pressure stimulus is directly applied to the sensor element underneath the capsule, the receptor response to a sustained stimulus substantially increases (Mendelson &amp; Loewenstein, 1964). Basically, these elastic lamellae slowly mold their shape to the stimulus and pressure stimuli are only transmitted for a few milliseconds, after which the lamellae absorb the pressure. So only a one or a few spikes are generated at the start of a pressure stimulus. 

You can imagine it this way, suppose you have a gel-like substance (say Play-Do) on a weight scale. Now suppose you press the Play-Do quickly and hold your finger steady at a certain point above the scale, within the Play-Do. The weight scale will shortly display a certain weight (equal to the pressure applied in Newtons), after which the Play-Do folds itself around your fingertip, and the pressure is released from the scale. 

Upon release of the pressure stimulus another spike or two are generated in the PC. Hence, a continued pressure stimulus is not faithfully reproduced. However, vibratory stimuli generate spikes on the pressure onset as well as offset as well, and this happens on every phase of the vibration. Hence, vibratory stimuli are faithfully transmitted.  


Response of Pacinian corpuscle to a sustained pressure stimulus (indentation of the skin) and superimposed vibratory stimuli. Note the vigorous response to vibrations, but the rapid adaptation to the static pressure stimulus. Source: Zavantag   

References
- Johnson, Curr Opin Neuobiol 2001;11:455–461
- Mendelson &amp; Loewenstein, Science 1964;3618:554-5