From what I see in the instructions, the manifold tube with the crimped end is designed for a technician to cut the crimp off. Then a compression fitting will be installed on that cut end. The fitting will have an opening suitable to connect to the supply pipe source.
I would imagine the same could be done on the opposite end with the black cap as well. (depending on which side is closest to the supply)
The cutting would likely be performed with a small tubing cutter appropriate for working in tight spaces like the one shown in this link.
Small tubing cutter
This site shows the layout & use of a compression fitting. (in this example both ends are using a compression sleeve) Although this is showing copper tubing, it is the same principle for clean steel tubing.
How to use a compression fitting
When using compression sleeves, it is important to be successful on the first attempt as the sleeve will be crimped tight to the tubing. If you fail you will have to cut the tubing again to remove that sleeve.
The site this link leads to shows connecting two pieces of bare tubing together. I don't know what your gas supply tube/pipe looks like so I can only answer in the abstract.