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Everything @MonicaCellion said is correct but to provide some more background information: The reason that cheese like Gouda, Cheddar and Tilsiter (and any solid cheese) don't melt the way you expect is because they are relatively dry. So trying to melt it in one piece won't really work.

Grating the cheese allows the respective surfaces of the bits to be relatively evenly and thoroughly warmed, leading to an actual melting process. I usually take milk or cream, grate the cheese and cook everything on a low heat until I have a thick cheese sauce. (Low heat so I don't burn the milk or cream.) The result is a thick sauce, clumps may also be visible.

As for the part about sodium citrate: If you can get something, you could indeed use it at home to achieve something similar to the experience you're missing right now - and the best part is that it doesn't matter kind of cheese you use.

[Some chemical background information about sodium citrate](https://foodadditives.net/acidity-regulator/sodium-citrate/): Sodium citrate actually comes in three "flavours", as mono-, di- and trisodium citrate. For cheesemaking, we're interested in trisodium citrate as it helps to keep everything together.

Cheese consists of three major components: fats, proteins and water which are bonded together and form cheese. As soon as you heat up cheese, these bonds can dissolve and break. Fats and waters now may separate from the proteins, leading to the aforementioned thick sauces with occasional clumps.

Trisodium citrate helps to fix this problem by changing the melting process: It prevents that fats, proteins and water separate from each other and helps to distribute these components equally in the to-be-used base component, e.g. milk or cream. The result is a cheese sauce, similar to the ones you can buy as dips in supermarkets.

A recipe for a do-it-yourself cheese sauce at home: https://www.foodrepublic.com/recipes/take-modernist-cuisines-nacho-cheese-sauce-for-a-spin/