Memes

At the point of putting Gruyère in it you better be calling it Macaroni au Gratin Mornay.

Following Escoffier (as is proper and yes I know these aren't home kitchen amounts do the ratios yourself):

For the Bechamel: Steam in butter until white 300g finely diced lean veal, two small finely chopped onions, a bit of thyme, pepper, nutmeg, 25g salt, add it to a base of 650g roux and 5l milk. TBH that's too complicated for me, I put meat extract, thyme, pepper, and onions into cold milk, slowly bring it to temperature, then add cold roux and nutmeg. Also I don't pass it through a sieve do I look like a French Chef chunks are fine.

To turn that into a Mornay, add fish fonds (fish sauce works well use the good Vietnamese stuff), for 1l Bechamel melt in 50g "Swiss cheese" and 50g Parmesan, 100g Butter. Gruyere has become standard for the Swiss part of the cheese and works on its own, often people also use an egg yolk to aid emulsion. Especially useful if you have less aromatic cheeses and want to add more, it's not like you can't do this with Gouda.

Oh, and you might want to reduce the salt in the Bechamel if you add fish sauce.

If you're putting the whole thing in the oven to make a gratin (also consider throwing some veggies in, peas, carrots, nothing special the sauce is already fancy) adding the egg white is fine no harm done and extra protein, otherwise things can get complicated in actually getting it denatured properly. Without producing scrambled eggs, that is: Mix in the yolk once the sauce is cold enough to not instantly denature it, melt in the cheese, now it's even colder, add the white and mix it well, pour over stuff, then into the oven to finish up.