Invalid R package checks

I am updating a package where a package depends on another package. The code worked under R 3.1.0, but not under 3.4.0.

So how can I complete the "DESCRIPTION" when my package dependency depends on another package (mgcv).

I want this dependency to be installed when my package is installed, if not already installed.

When I read "Writing an R Extension" I should write:

Depends: mgcv

in the file "DESCRIPTION"

In R 3.1.0 it worked fine, but now when I want to upgrade my package to R 3.4.0 and run a check. I am getting the error:

** checking package dependencies ... ERROR Required package but not available: "mgcv" See "File DESCRIPTION" in the Writing R Extension Guide. *

Any help is really appreciated