As a professional courtesy, GAO will inform requesters of substantive media inquiries during an ongoing assignment.
( ( As ( a ( professional courtesy ) ) ) ( , ( GAO ( ( will ( ( inform ( requesters ( of ( substantive ( media inquiries ) ) ) ) ) ( during ( an ( ongoing assignment ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN As) (NP (DT a) (JJ professional) (NN courtesy))) (, ,) (NP (NNP GAO)) (VP (MD will) (VP (VB inform) (NP (NP (NNS requesters)) (PP (IN of) (NP (JJ substantive) (NNS media) (NNS inquiries)))) (PP (IN during) (NP (DT an) (JJ ongoing) (NN assignment))))) (. .)))
It is rare for GAO to do something as a professional courtesy. 
( It ( ( ( is ( rare ( for GAO ) ) ) ( to ( ( do something ) ( as ( a ( professional courtesy ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (ADJP (JJ rare) (PP (IN for) (NP (NNP GAO)))) (S (VP (TO to) (VP (VB do) (NP (NN something)) (PP (IN as) (NP (DT a) (JJ professional) (NN courtesy))))))) (. .)))