The picture below aims at summarising an agile SDLC combined with a stage gate process and, using the cone of uncertainty, why having regular gates based on the velocity of the team and the business case updated (ROI and risks):
Governance, Project Management
Agile SDLC (ALM), Cone of uncertainty and stage gate process
christophe le coënt 
Hello Christophe, once again I’ve translated into french one of your interesting posts :
http://agilarium.wikispaces.com/Gestion+du+cycle+de+vie+des+applications
Thank you, Fabrice
Merci Fabrice!