Aww I totally understand your worry lah, especially when baby already has an underlying heart condition, every little thing also make us extra anxious as mummies! But it does sound like what the doctors said is right, the morning vomiting after long sleep is quite common because phlegm accumulates overnight and when baby coughs to clear it the gag reflex triggers the vomiting, and at 6 months with solids introduction the gagging and occasional vomiting is also very normal as they are still learning to handle different food textures. Since you have already seen multiple doctors including a gastro specialist and they are not concerned, try to take some comfort in that lah, just continue monitoring and if the vomiting frequency increases significantly, baby loses weight, or you notice any changes in his heart condition symptoms then go back to see his doctors immediately, you are doing a great job being so attentive to your baby