If you had to explicitly add 'nomodeset' to your kernel command line then that suggests the problem is that Linux detected a KMS-capable GPU still, and thus GDM would try to use it by default. Is it possible your i7-4790K has its integrated GPU still enabled (in your BIOS), or that you have other graphics cards installed?
Thanks.
If you had to explicitly add 'nomodeset' to your kernel command line then that suggests the problem is that Linux detected a KMS-capable GPU still, and thus GDM would try to use it by default. Is it possible your i7-4790K has its integrated GPU still enabled (in your BIOS), or that you have other graphics cards installed?