Is liquid seeping from the fish normal when making gravlax?
Yes it is normal.

The salt (and sugar) will "extract" the water from the fish meat.

[...]On a technical level, what both the salt and sugar do is draw moisture out of the fish through osmosis. This decreases the moisture level of the fish, which in turn makes it less hospitable to microbial life. The salt, meanwhile, also helps ward off bacteria that would otherwise hasten spoilage. This extends the edible life of the salmon, but only for a short amount of time—gravlax is not cured in any long-term sense of the word. [...]

Taken from : http://www.seriouseats.com/2015/04/how-to-make-gravlax-cured-salmon.html