Besides, the only reason Eccleston's contract was only one season with an option for additional was because no one knew if the show would be a success or not. Thinking there was a possibility the show could only last one season, they weren't going to make Eccleston commit to any other seasons which might not happen, but left the option for an additional open as a means of not shutting out any possibilities. All his successors have had multiple year contracts. Tennant was under contract until 2010, Smith, Capaldi and Whittaker each had three year contracts. I'd be very surprised if Gatwa didn't have a three year contract. He would not have known when he was signed on that his popularity would take off as quickly as it has and three years guaranteed as the lead of one of Britain's most internationally renowned TV properties would have been too good an offer to turn down at the point his career was in last year.
Besides, it would be bad optics if the first black Doctor to be the show's lead were also the first one since 2005 to only be signed on for one year.