When Gill was made captain, people already suspected that he might lose his form and this is what exactly happened. In IPL 2023, Gill was orange cap winner, and his tons played large part in GT's wins.
If Hardik was still the captain then I assume he would have still played good as he did last year, now that he's himself a captain he'll have to get accustomed to captainship so it may take a while to do both duties properly.
Shubman Gill was made the captain because they didn't had anyone else who is suitable for that job. Kane Williamson is there, but he was unavailable for most of the matches. Then there is Wriddhiman Saha who is close to retirement at 39 years. Another option is Rashid Khan, but he also doesn't have much experience in captaining IPL franchises. What happened to Gill in terms of his form was unfortunate. But I still believe that the GT management did the right thing here.
I'm not saying Gill is bad choice or anything, he's fine, it's just that it may take a while before he can manage both batting and captainship responsibilities.
I'm not sure if Williamson was unavailable, he was there but wasn't given chance in squad. On Saha, bit rude but Saha doesn't deserve to be in squad, rest alone captainship. I'd put Rashid as in same league as Gill but ofc domestic talent takes lead over overseas ones.