There is no meaning attributed to the intensity of a line. Even synthetic urines (which contain no drug metabolites at all) can register a faint line on an assay.

The GC/MS is only used to confirm positives that have occurred on the intial assay screen. It would be too costly for both the lab and the client to use the GC/MS as the primary analysis.

Dont let the lower cutoff of the GC/MS worry you. A 50 ng assay is functionally equivalent to the 15 ng GC/MS. Their cutoffs numerically differ is because the assay recognizes all 31 metabolite concentrations combined, while the GC/MS only recognizes one metabolite concentration (THC-COOH).

In order to test negative on the assay, the whole combination of the 31 metabolite concentrations must register below 50 ng.
In order to test negative on the GC/MS, only that one metabolite concentration, THC-COOH, must register below 15 ng.