If someone replies to a message sent by, e.g. support@isk..., the reply will actually be sent to the gmail account, but if they initiate a message to support@isk..., not a reply, the message won't be received if the support@isk... address isn't an actual mail account. That is why I suggest the 'identity' addresses be connected to working accounts, and then, if you like, have gmail collect all the mail sent to those addresses, not just the ones sent as replies.

The problem here is I have successfully added the @isk.. mail to my gmail but it does not receive mails to @isk.. unless the email is verified which cannot be done as the alias username does not exist.

An alias is just a made-up name, and a message sent to the alias address won't arrive if the address is not connected to a real mail account. In your picture, you have added an @isk.. address as an allowed sender, but this is for when you want to send through the gmail smtp, but have the recipient see the @isk.. address as the sender, not the gmail address. You already have the @isk... sending through pepipost. I think you want to add the @isk address in 'Import mail and contacts' to have gmail collect mail sent to @isk, but this will only work if @isk is a real mail account, not just an alias.