My ex also has ADHD (diagnosed as a child, and again as an adult) so we didn't let DD have screen time at all until she was 2.5. Sadly, she's still about the most ADHD kid you ever met, but I'd say she came by it fairly enough given her genetics. All that said, it certainly never hurt her to not have access to screened devices as a young child. I can't actually think of any way it would have benefited her to have screen time. From 2.5 to 3.5 she got to watch TV about half an hour per day, and since then her interaction with TV and computers has really been low and sporadic. However, minimal screen access has not prevented her from learning age-appropriate computer skills.
FWIW mobile devices are so incredibly user-friendly that my son learned to fully operate our android tablet in a few hours, just after turning 3, before he could read. He also had no screen time until after age 2, and has only since age 3.5 been allowed to use the computer sporadically to access edutainment.
I honestly don't feel like gifted kids have any greater need to access screen time than other kids most of the time. On days my daughter (then 7) was not in summer camp and I was working at home last year, it was handy for me to let her play on dreambox or codeacademy for a couple hours at a time. But that was all about the circumstances at play.
