GPS L on FMC
 

Does anyone have information about why we choose GPS L on FMC on preflight procedure?

I will assume that yours is a Boeing question, if it ain't Boeing, I ain't answering. On the 737, if I recall correctly, it's SOP. Remember not to deviate from SOPs unless there's a good reason, which in this case, I'm unable to provide.

I don’t think it has a technical reason as the antenna are in the same place.
In the Western world we write from left to right and count from left to right (and top to bottom)
So the left engine is #1 and so is the GPS-L.
Afaik when the GPS drops out it’s the IRS-L that provides position until radio nav updating is selected.