It’s not so clear in the docs but you can use the GammaCorrectionShader like so:
const gammaCorrectionPass = new ShaderPass( GammaCorrectionShader );
composer.addPass( gammaCorrectionPass );
Keep in mind it’s not exactly like outputEncoding = sRGBEncoding, though, because Fog and Scene.background need to be manually converted to linear color space otherwise they’ll look incorrect.