It's common for fans to think players are worth more than their actual trade value.
The reality is Gill is 31, experienced a major injury, was a holdout, no TC, hadn't played this year and in 2020 his play was good but had declined from 2019.
The time to trade him was during the draft in 2020 but with a lower cap number is was harder to do so plus Gilly wanted more money. I'm assuming because the offers were lower than what BB was willing to accept, he decided to just keep him as he was a DPOY/All Pro.