Get GeoJSON Features using Mapbox?

You can use querySourceFeatures which return all geojson features within a layer if no filter is applied.

var features = map.querySourceFeatures('your_layer_id');

https://www.mapbox.com/mapbox-gl-js/api/#map#querysourcefeatures

But you may check first that the layer is fully loaded, or you will get an empty array.

map.on("sourcedata", function(e) {
    if (map.getSource('your_layer_id') && map.isSourceLoaded('your_layer_id')) {
        console.log('source loaded!');
        var features = map.querySourceFeatures('your_layer_id');
        console.log(features);
    }
});

Source about the event: Mapbox GL JS - "Source Loaded" event