G-Works adapter screws on to the top of an everyday ordinary Coleman type 100% propane canister, the kind that are used for car camping, picnics, BBQ's, etc.  The other side of the adapter is a 7/16" UNEF threaded connector – just like the one on the top of a backpacking type gas canister.  You screw in 100% propane on one side of the adapter and your backpacking stove on the other, and, voila! – you're cooking on 100% propane.
 
Now, the big 16.4 oz/465g 100% propane canisters are pretty heavy and bulky.  Why use them?  Well, see my list of reasons, below.

However, note, that there are a lot of techniques and technologies that can help you use regular backpacking canisters in cold weather; see Gas Stoves in Cold Weather – Regulator Valves and Inverted Canisters.  You can also use a liquid fueled stove in cold weather.  This adapter is just another option.  Each person should evaluate his or her own circumstances and preferences and choose accordingly from among the options.

Regarding the weight of the 100% propane canisters:  Coleman brand 465g canisters of 100% propane. They weigh about 850 g each. The weights vary, but the lowest seen for a full 100% propane canister is about 840g. By contrast, a 450g Jetboil brand backpacking type canister weighs 660g.  That's a minimum difference of about 80g (roughly 3 oz) for comparably sized canisters.  Some Coleman 100% propane canisters weigh 870g.  That's about about 110g (about 1/4 pound) more than a comparable backpacking canister.

Is the weight worth it?  Well, that's something you're going to have to decide for yourself.  Read the rest of the post and see if benefits are sufficient to justify the weight of the heavier canister.
 
Reasons to use this Adapter
1.  Cold weather - Why 100% propane?  Well, for starters, cold weather.  Butane, which is often the majority component in backpacking gas canisters, vaporizes at 31F/-0.5C.  That's really not all that cold.  Not only that, but you have to be about 20 Fahrenheit degrees (10 Celsius degrees) above the vaporization point before you have consistently good pressure.

Propane on the other hand, vaporizes all the way down to -44 Fahrenheit/-42 Celsius.  That's cold!

Vaporization (Boiling) Point
 n-butane    -0.5°C    31°F
 isobutane    -12°C    11°F
 propane      -42°C   -44°F
 
And yes, I know they usually blend in some isobutane and propane into the typical backpacking gas canister, but still, nothing beats 100% propane for cold weather.

NOTE:  If you're going out in really cold weather, TEST YOUR SET UP FIRST.  Propane is typically not pure (well, maybe if you work in a laboratory or something).  The temperatures listed above are based on pure propane which you can't buy at least in the US, so you're not going to get quite the good cold weather performance that you might expect.  The propane typically sold in the US conforms to the HD5 standard which is as follows:
 
Minimum 90% propane (no less than 90%)
Maximum 5% propylene (no more than 5%)
The remainder is comprised of "other" petroleum based gasses
 
The "other" might include methane, ethane, isobutane, butane, etc., some of which will give you less performance in cold weather, others of which will give you better performance.  So, the number -44F/-42C is something of an approximation, depending on exactly what you've got in the canister.  Still, 90% minimum propane is going to be significantly better than any backpacking canister's mixture which is typically no more than 30% propane.  And remember that you need to be about 20 Fahrenheit degrees (10 Celsius degrees) above the vaporization point before you have decent pressure.  Of course, one can always warm the canister by various means, and so long as isn't warmed to the point of being too hot to touch, it should be safe while giving one good pressure.


2.  Instantly "winterize" your existing stove - With this adapter, you don't have to shell out the cash for a dedicated winter stove.  Here's what I mean:  Say you're basically a summer camper.  Most people are.  But every couple of years you get a wild idea and go out when it's cold.  A stove is pretty close to a necessity on a winter trip.  But do you really want to spend the money on an expensive stove that you're only going to use every couple of years?  No, of course not.  No problem.  Just get this little adapter, and voila! you instantly have a winter capable stove.

3.  Outlying areas (no backpacking canisters available) - In a lot of outlying areas, specialty items like backpacking canisters may simply not be available.  I know guys who have hiked in rural New Mexico where they just couldn't find backpacking canisters, but, walk into a hardware store or gas station, and, there they are:  100% propane car camping type canisters.  Having this little adapter might mean the difference being able to do a given trip – and having to just go home.  This is particularly true if you have to fly in for a given trip and cannot take canisters from home.

4.  Natural Disasters - You could look at this adapter as $20 worth of cheap insurance.  This adapter opens up a whole new fuel supply for your backpacking stove.  The ability to boil water for drinking can be critical in times of natural disaster.

5.  Cheap fuel - Another reason for buying 100% propane is that it's typically cheaper.  I've seen propane for as cheap as $2.50 (USD) per canister.  About the cheapest you'll find for the equivalent amount of backpacking gas is $7 or $8.  That's a HUGE price difference.  Why?  Well, think about it.  For every backpacker, there are hundreds of car campers, hunters, picnickers, and back yard barbecuers.  The economies of scale just aren't there for backpacking canisters.

6.  Trailhead camping – or just using your backpacking stove for everything.
You might also want to bring 100% propane to cook your supper and/or breakfast if you spend the night before a backpacking trip at the trailhead.  That way, you start with a 100% full backpacking canister for your hike.  Or heck, maybe you just want to own one stove, and your backpacking stove is it.  You use backpacking canisters when backpacking, but when car camping, picnicking, etc. you just use cheap propane, and who cares about the weight when you're using your car, right?