Chain the actions and callbacks so with the final callback is the final function called in the chain. fw -> f1 -> fw -> f2 -> fw -> f3 -> callback

chain-callback-func-list.js

var gtmActions = {
    "a": function (callback) {
        console.log("a");
        window.setTimeout(callback, 500)
    },
    "b": function (callback) {
        console.log("b");
        window.setTimeout(callback, 500)
    },
    "c": function (callback) {
        console.log("c");
        window.setTimeout(callback, 500)
    },
    "d": function (callback) {
        console.log("d");
        window.setTimeout(callback, 500)
    }
}

function wrapCallback(fn, callback) {
	return function () {
		fn(callback);
	}
}

var actions = ['a','b','c','d'],
    fn = function () {
        console.log("done");
    };
while ((a = actions.pop())) {
	if (gtmActions.hasOwnProperty(a)) {
		fn = wrapCallback(gtmActions[a], fn);
	}
}
fn();