Why would the Inventory module not available in Reconcile to GL window on Dynamics GP 2013? Look at the HITB report.
Inventory module not available in Reconcile to GL window on Dynamics GP 2013 – Dynamics GP Support and Services Blog – Microsoft Dynamics GP – Microsoft Dynamics Community
General Ledger & FinancialsMicrosoft & the GP EcosystemSales, Purchasing & InventoryGP 2013
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.