GraphDB's similarity index leverages the SemanticVectors library to build vector representations for semantic similarity searches using the RDF graph structure and metadata. However, it does not natively support the integration of external embedding models, such as OpenAI's embeddings, for vector creation within the similarity index framework.

But in GraphDB 10.8 version we've introduced https://graphdb.ontotext.com/documentation/10.8/talk-to-graph.html#talk-to-your-graph feature which probably will be useful for your usecase.