Hi Ryan, welcome to CS.
It sounds like what you're experiencing is the operational difference between a steam thermoblock (breville) and steam boiler (gaggia).
Thermoblocks produce steam by the pump 'pulsing' small volumes of cold water through channels in a heated metal block, which flashes the water to steam instantly allowing more or less continuous steam.
The Gaggia however heats a larger volume of water inside the boiler to produce steam, which is then gradually exhausted when you open the steam valve. The important part here is that the pump doesn't run to re-fill the boiler during steaming, so sooner or later all the water will be expelled and you'll be left with hot air until there is more water brought in from running the pump.
Now, 20 seconds of steam sounds a bit extreme. One thing to make sure is that the boiler is full before you start steaming, by running the pump as if you were making coffee, till a steady stream of water flows from the group. You should then get enough to easily heat up a small jug of milk, let's say 2 minutes. One more thing to try is to open the wand while the machine is still heating up in steam mode (say when you get some steam come out of the group). That way you trick the thermostat into staying on longer, giving you more steam pressure.
If this doesn't improve the situation, there may be an issue with the thermostat. You should get very powerful steam in the first few seconds after you open the valve. I would leave descaling to a last resort and suggest it might be just a different technique required to get good steam from the Gaggia.