Global investment in artificial intelligence increasingly relies on externally sourced AI systems, foundation models, datasets, and AI-enabled services. While regulatory frameworks and organizational governance mechanisms continue to evolve, most enterprises lack an enforceable ex-ante decision point at the moment when AI-related risks are contractually accepted. This publication introduces the AIGN Procurement Governance Gate as a formal decision and enforcement mechanism governing the procurement of third-party AI, models, and data. The gate operates as a mandatory pre-contract control point that consolidates governance inputs across legal, risk, security, and ethical domains into a single, authoritative decision outcome (Go / Conditional Go / No-Go). Embedded within AIGN OS – The Operating System for Responsible AI Governance, the Procurement Governance Gate operationalizes governance at the point of purchase through defined trigger conditions, structured risk classification, enforceable contractual safeguards, and an audit-ready decision artefact. By shifting AI governance from post-hoc oversight to ex-ante authorization, the gate closes a critical procurement governance gap and enables organizations to demonstrate accountability, auditability, and regulatory alignment across jurisdictions, including the EU AI Act and international AI governance standards. This publication presents a governance infrastructure artefact rather than an operational tool or checklist and contributes to the emerging field of systemic AI governance by formalizing procurement as a core governance layer.