Preparation of gravlax versus lox

I'd say you probably brined the salmon for too long. Gravlax goes generally from 12 to 48 hours in a dry brine.

You have to know that

• The longer the salmon is left in the brine the more it gets cooked and firm. Try shorter brine time (12-48h).
• The more saltier the brine is, the firmer the salmon will end up. Try increasing the sugar ratio in your mix.

If the result is uneven, you should flip the fish each 12 or so hours. After the brining and removing all the salt under running water, let the fish rest (dry) for a few hours to let the flavors balance inside the fish.

For the smoking, you should take care of cold smoking it (fish temperature should be kept below 37°C [100°F]) as to maintain gravlax properties. However as it was mentioned in the comments, it should then be called smoked salmon instead of gravlax as the gravlax is generally not smoked. Inserting a thermometer in the fish is helpful to control temperature during the whole process.

A trick to maintain gravlax properties and confection, but to add some smoke flavor is to add some smoked tea to the brine.

You can use liquid smoke from a butcher supplier and use the traditional method of not smoking.

Every cook has their own recipe. The Gold standard for Jewish deli Style Belly Lox is the ACME lox product. Folks gravitate toward NOVA these days, not lox, but smoked salmon product. Gravlax is never smoked. It is just brined. I make let my gravslax sit on the salt/sugar mixture 2-4 days (usually 3). Commercial makers use a ice water brine. For home we just use salt (lox) or 50/50 salt and sugar (gravlax). I think my belly lox is as good as the ACME product. The only ingredient they list on the label is salt.