Hi SilverBird
First let's define entity as Game Guru see's it. In general there are two types of entities, Characters (AI), and non-character entities like buildings, cars, tables, etc. I am unsure of your 3D skill levels. Characters (new horror zombie) are very complex and require above average skills to create. Easier, but only a bit less complex are models which can be created in a variety of software systems.
Many GG'ers use Blender (a free but very comprehensive 3D system) to create models which can be imported into GG. You'll also need to deal with the application of textures. Game Guru uses a specific set of textures. You can search the forum for these specifications. Once the model and textures are combined, they can imported into GG and used in game creation.
So the question is; do you have these skill sets, or are you willing to spend the time to learn them?
Either way.. good luck
"THERE IS NO SPOON"
AMD 6300 6 core 3.5 ghz, Windows 10, 8GB ram, GTX 1060 2GB ram