The 46-year-old actress - who is expecting a baby with fiance Olivier Martinez - already has five-year-old daughter Nahla with Gabriel and although they have had an acrimonious relationship since their split, the Canadian model is happy his daughter will have a new half-sibling.
A source told RadarOnline: ''Gabe knew that Halle had been trying to get pregnant for a very long time, and Nahla is very excited to become a big sister. Seeing Nahla happy is all that matters to Gabe.
''Nahla will be starting kindergarten in the fall, and Halle and Gabe are in the process of selecting a private school for her.''
Although Halle was denied by a judge when she tried to move to France with Olivier and Nahla, Gabriel isn't worried she will try again.
The insider added: ''Gabe isn't concerned that Halle will try and move to France with Nahla because she got shut down in the custody trial. The judge made it clear, Halle can move wherever she wants, but Nahla stays in Los Angeles where Gabe lives and she has been raised. Gabe doesn't think Halle will try it again, but would absolutely fight any attempt to move Nahla out of Southern California.
''Gabe has been a doting dad to Nahla, and has joint custody of her. Gabe knows that Olivier loves his daughter, and isn't worried about her feeling left out when her little brother arrives later this year. Halle and Olivier can raise their son wherever they want, but Nahla stays in Los Angeles, with her dad, end of discussion.''