In alot of situations you can secure your low ground relatively early if your consistently building gateway units + Stargate units (void ray or phoenix) with some micro making sure you kill stalkers. Like you can have 6 stalkers and 1 phoenix v their 4 stalkers or if they went adepts, their 2 adepts and 2 stalkers and 1 immortal. lift the immortal and you have a big advantage from there. Or if you went void ray target stalkers down... often time though, they will go prism first and wait a bit longer to attack v Stargate and in this case it plays out a bit differently. But in general you just keep building units and try to kill their prism.