The SecInfo -> CVEs view within the GSA isn’t related to the availability of VTs for a specific vulnerability.
This database is filled / updated by the “SCAP” feed shown in About GVM Architecture and is based on e.g. the data feed provided by https://nvd.nist.gov/vuln/data-feeds.
For this specific vulnerability the severity was assigned by the NVD yesterday:
https://nvd.nist.gov/vuln/detail/CVE-2020-1350#VulnChangeHistorySection
There might be 1 or 2 days delay (depending on when this change is available in the NVD data feed), afterwards the severity will be shown instead of the N/A.
Generally it might be also possible (unrelated to this specific CVE) that a CVE is shown as N/A for more then a few days or even a few weeks depending on when the NVD is doing the vulnerability analysis for a specific CVE. Such CVEs can be identified when browsing the related entry like https://nvd.nist.gov/vuln/detail/CVE-2020-1353. As long as the Undergoing Analysis is shown there the CVE will show up with a N/A severity within the GSA:
As a side note:
If you want to check for the availability of specific VTs to be used for a scan you need to check the SecInfo -> NVTs item within the GSA.