He broke wind whilst doing ghusl from janaabah; is his ghusl invalidated?

Passing wind is one of the things that invalidate wudoo’ but it is not one of the things that invalidate ghusl. Based on this, if a person touches his private part, urinates or passes wind whilst doing ghusl, he should complete his ghusl, then do wudoo’ afterwards. 

The basic principle is that if ghusl is done following major impurity, then it suffices for wudoo’. If a person is junub (impure following sexual activity) and he does ghusl, then that suffices for wudoo’, because Allaah says (interpretation of the meaning): 

“If you are in a state of Janaaba (i.e. after a sexual discharge), purify yourselves (bathe your whole body)”

[al-Maa’idah 5:6]

He does not have to do wudoo’ after doing ghusl, but if one of the things that invalidate wudoo’ takes place whilst doing ghusl or afterwards, then he has to do wudoo’ for prayer. But if he does not break his wudoo’ then his ghusl from janaabah suffices for wudoo’, whether he did wudoo’ before ghusl or not. But it is essential to pay attention to rinsing the mouth and nose, because these are essential for both wudoo’ and ghusl. 

Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 11/228 

And Allaah knows best.