Yes, you can use your existing Lazada account to shop on Lazada in the GCash App. However, you must make sure that any GCash account linked to that Lazada account should be the same GCash number where you are accessing Lazada. If there is a different GCash mobile number linked to that Lazada account, you will not be able to sign in.
Yes, you may use any Lazada account to shop on Lazada in the GCash App as long as the GCash number linked to it is the same as the GCash number you are using or there is no currently linked GCash account.
If another GCash account is already linked to your Lazada account, you will not be able to proceed with logging in. To proceed using your current GCash App, you will have to unlink the other GCash account from your Lazada account.