Where the total weight is indicated: total weight – result of weighing = weight of gas used.
Where the empty weight is indicated: result of weighing – empty weight = weight of gas left.
How long will the remaining gas last?
The answer will depend on your rate of consumption. If your gas appliance gives an indication of grams per hour, you can easily calculate the remaining duration of gas consumption. Usually, however, no such indication is given, in which case you will be forced to rely on experience: if you know roughly how long a full canister will last on average, you can work out the remaining gas as a percentage of the whole canister and base your estimate of the remaining duration on that.