Both dry electrodes and gel electrodes are commonly used for EEG, ECG, EMG, SCR measurements. Dry electrodes are durable, easy to use and easy to clean, while adhesive gel electrodes generally make more stable skin contacts, but may need replacement after repeated use. A good "hybrid" solution is to apply a little (adhesive or non-adhesive) conductive solution or gel to dry electrodes for more stable skin contacts. As our human body is basically water with a little salt (in technical terms: "saline solution" and "electrolyte"), most conductive solution or gel basically contain water with a little salt (see 'how to make DIY conductive solution and gel.') Most of these conductive solution and gel can be easily washed in water, so cleaning is quite simple.
Depending on which bio-signal you are taking, different requirements for "good" skin contact may apply. Since the input amplifier has very high input impedance compared to typical skin contact resistance, generally the skin contact resistance can vary a broad range without affecting much the measured signal. However, "poor" skin contact is usually caused by or associated with unstable or intermittent contacts, and the "motion artifacts" created by unstable or intermittent contacts can often overwhelm the bio-signals being measured. As the motion artifacts are generally in the lower frequency range (<4Hz), it has greater detrimental effects there. EEG generally has smaller signal and requires good skin contact, in applications where lower frequency (<4Hz) are filtered out or reduced, dry electrodes can be quite adequate, and added gel can be even better. ECG (or EKG) generally has larger signal, and dry electrodes are often adequate. SCR (or GSR) monitors the skin conductance itself, and needs more stable and consistent skin contacts. Furthermore, motion artifacts are generally difficult to be separated out from genuine SCR signals. Gel enhanced electrodes are advised for SCR measurements.