Brainlabs analysed Google Analytics data from fifty-four clients following the rollout of AI Overviews from September 2025. Organic sessions fell on average from 140.1 million to 125.4 million — a 10.5 per cent decline; individual sites saw organic cuts of up to 58 per cent. AI platform referrals from ChatGPT, Gemini, Microsoft Copilot, and Perplexity grew 163 per cent, reaching approximately 200,000 monthly sessions on average. AI-driven conversions rose 335 per cent, and the key-event rate for AI referrals was 1.5 times higher than for organic traffic. Sector breakdown: fitness, fintech, insurance, and CPG took the largest organic hits; retail, beauty, and entertainment saw minimal declines. ChatGPT dominated AI referral share throughout the measurement period. The data provides the first defensible sector guide to GEO investment urgency: brands in high-organic-loss verticals are already experiencing the structural shift; those in lower-impact verticals have a narrowing window before the same pattern applies.