Millwall boss Willie Donachie has agreed to sign striker Gary Alexander and midfielder Danny Spiller, who will join the Lions on 1 July.
Spiller and Alexander will put pen to paper at Millwall on 1 July
Alexander, 27, opted to leave Leyton Orient at the end of his contract.
"Once I knew Willie wanted to sign me, there was only one club I was going to join," he told Millwall's website.
Meanwhile, 26-year-old Spiller - who had also been linked with Millwall's neighbours Charlton - rejected the offer of a new contract at Gillingham.
"I'm ambitious and I believe Millwall are going to be strong challengers for promotion next season," Spiller added.
Both players have completed medicals at the New Den.