Making a dummy plug so you can run your GPUs without attaching a monitor is very simple and easy to do.
Here are two versions of a dummy plug that achieve the same result:
This example offers ease of use and durability:
The 30 second Dummy Plug
This example is best when removal is not required:
The 5 second Dummy Plug
If you can't find the exact resistors mentioned above, 100 Ohm 1/4 watt resistors work just fine.