I’m having the exact same issue. I’m not sure that it’s an openssl problem either since sending a raw request using openssl works:
printf "POST /v1beta/openai/chat/completions HTTP/1.1\r\nHost: generativelanguage.googleapis.com\r\nAuthorization: Bearer $API_KEY\r\nContent-Type: application/json\r\nContent-Length: 96\r\n\r\n{\"model\":\"gemini-1.5-flash\",\"messages\":[{\"role\":\"user\",\"content\":\"Explain to me how AI works\"}]}\r\n" | openssl s_client -quiet -connect generativelanguage.googleapis.com:443