Dry, wet or semi-dry electrodes: gel establishes the electrochemical interface. Without it, you need an active buffer stage to handle the impedance.
We speak of dry electrodes as a convenience, yet conductive gel is not an optional consumable: it is what creates the electrochemical bridge between skin and metal. Removing it changes the physical nature of the measurement.
Three interface regimes
The wet electrode floods the contact in an electrolyte that stabilizes the potential and lowers impedance to a few kiloohms. The semi-dry releases a small amount of electrolyte on demand, a compromise between comfort and duration. The dry electrode relies on a capacitive or resistive contact at very high impedance.
At high impedance, the slightest amplifier bias current creates a huge error voltage. That is where the dry electrode fails without dedicated electronics.
The sequence is reversed
Dry only truly works with an active buffer stage built into the electrode: a follower with very high input impedance presents a low-impedance signal to the cable. The sequence is reversed from intuition, the buffer frees you from gel rather than the gel freeing the buffer.
The takeaway: before promising gel-free, ask where the active stage is. Without it, dry is not an electrode, it is an antenna.