The d6 is about 11" deep. The Gemini is less than an inch and a pound or so lighter.
Just to keep the facts accurate, the D6 is 6.85" behind the panel, not 11". It weighs 1.56 lbs. TruTrak doesn't publish weights in their install manuals so I can't tell you how different that is. The TCW backup battery weighs 1.5 lbs while ours is 6oz, so if you are backing up your Gemini, the weights are likely identical.
The D6 and the Gemini are pretty different products. The D6 has the ability to display attitude, airspeed, altitude, turn rate, vertical speed, and actual magnetic heading. These functions all work the way you'd expect from the gauges in a standard 6 pack in any airplane you have flown.
The Gemini does not display attitude or heading. The display of attitude is actually vertical speed and turn rate, and is limited to lower bank and pitch angles. The display of heading is GPS ground track. The D6 has no attitude restrictions and is fully aerobatic. It is also proven over time and temperature, with thousands sold and a fully specified temperature range from -20C to 50C.
As mentioned, the D6 can also do AoA if you have our $200 pitot probe (which also works as a standard pitot), and it can generate audio alerts for critical AoA. It can also be the encoder for your transponder.
Additionally, the screen on the D10A is much bigger and has excellent off-axis visibility, which is critical if it isn't in front of you. The D6 screen is fully dimmable for night flight, all the way down to about 1 nit. The D6 also includes everything you need to update the firmware right in your plane in the standard price, and has an internal back up battery that goes right inside instead of needing to buy a much more expensive external battery. In fact, when you add it all up, the prices are only $5 different.
Both Dynon and TruTrak make good products, so you'll get a useful, quality product either way. It's just important to know what you're purchasing for your backup and what it's features and limits are, and if they fit your needs for your aircraft's mission.
Good luck with your decision!
--Ian Jordan
Dynon Avionics