The G and Mk series are identical when it comes to dive related firmware features as well as most non-dive features. However, the G series does not include a model which supports wireless tank pressure monitoring via their transmitter. The G series also does not include the Wi-Fi connectivity, mapping, and music storage features included with the Mk series.
The more significant differences are in construction and styling: While the Mk series is adapted from the fēnix® series, the G series has a more purpose built design incorporating leakproof inductive buttons. The bezel and back of the Mk series are stainless steel vs the all fiber-reinforced polymer of the G series. The Mk series offer a full color display, while the G series is a monochrome display. These differences make the G series a significantly lower price than the Mk series. The G series is also available in a SOLAR version that may substantially increase battery life between charges. The G1 SOLAR model includes a three piece QuickFit-22 wrist band similar to three piece QuickFit bands found on the Mk series, while the non-solar version has a two piece integrated hinged band that can be removed with a special tool for compatibility with variety of optional QuickFit-22 two or three piece bands.
Please see Garmin Descent Series Comparison (235 KB PDF) for a detailed reference chart of all the differences.
