Hi Javier and Brock,
Before importing GCPs you need to select the coordinate system of the GCPs. You can select the coordinate system of the GCPs when creating the project or in the GCP/Manual Tie Point Manager.
Then you need to mark the GCPs on the images. Depending on if you have image geolocation and/or GCPs in a known or arbitrary coordinates system you have to follow one of the 3 methods. Have a look at the following article for the different methods on how to add GCPs to a project:
Note that if you are only interested in making measurements in the project and you are not interested in the absolute accuracy of the project (accurate position on the Earth) you can also use the scale constraint tool to give scale to the project.
With this tool you have to know precise distances in the model and you can use these distances to give scale to the project. For more information on how to scale a project: