I must admit that I haven't really figured things out about the gas, yet. I just notice that a lot of protoss get their gas up earlier than me, which is something I probably need to work on. The most important thing to scout in the protoss base (in my opinion and mostly my experience is up to gold, as I haven't played many games in platinum so far) is whether he builds a forge before a gateway, in which case you need to watch out for a cannon rush or whether the base is empty, which mostly means that he is building proxy gateways with chrono boosted zealots near your main. In that case, you can wall yourself in and build one cannon to hold that early attack, while also getting gas. Afterwards you pretty much have the game as you can easily kill the zealots and the proxy gates once you have stalkers.
Against terran it is important to see how many barracks he is building. If he doesn't follow up with an orbital command, but uses that 150 minerals to build another barracks, you are facing some early pressure and must be prepared to hold that. Still, against Terrans I would expand on 15 before building your gateway. It is important to get gas quickly, though, as you need a stalker to deal with marines, if he is able to micro them. In lower leagues, zealots are also fine against marines, but as of higher silver, people can typically kite the zealots with their marines.
All the best,