I wonder if activities like contract and agreement reviews, meetings to discuss preliminary information, SOW review, and contract approved (as deliverable and milestone) should be included in a gantt chart in a specific task list/group to be presented to an external customer or we should start our planning after our contract has been approved and skip this detail in the gantt chart.

I think that there are many different styles, contexts, and scenarios. I would like to know the general rule for these activities.

Once you define your baseline, will you include the reviews to ammend the contract, tracking all the meetings with the client?

In case you include all these activities, your start date for your project should be after contract approval, but gantt would make reference to date that we submitted our proposal.