AI Visibility Index: which agencies do ChatGPT, Perplexity and Gemini recommend?

Open measurement of how ChatGPT, Perplexity and Gemini answer buying questions about agencies for AI search visibility (GEO) in German and English. Baseline of 525 measurements (September 2026) and bi-weekly pulse measurements, with named providers and cited sources. Live index with measurement diary: https://www.grovia-digital.com/en/geo-visibility-index/