The V2G overall score is an aggregation of the different scores across sources and as you point out, it is not part of the variant_gene table. We pre-compute these values in order to display them in the FE. If you are curious in the score for a particular variant/gene, you can use the genesForVariant endpoint of the API to fetch the overallScore .
If you are interested in a larger set of variants, the scored V2G dataset can be derived from the V2G one we make available on BigQuery. To make so, the logic is available in our Genetics ETL here.
I hope that it is helpful! We are currently working towards harmonising the Genetics infrastructure with the one we have for the Platform, so we very much appreciate any comments that help us to make the data more useful.