The 20-year-old midfielder spent time on loan with both clubs last season, managing a total of 21 appearances in League One and League Two throughout the campaign.
When asked how it was to be a part of senior teams, he told Chelsea's official website: "That's the one thing I love about playing men's football because most people would say I play with my heart on my sleeve and play with a lot of passion, so it was good to be playing with the same kind of players.
"The Gillingham and Plymouth fans were amazing. Plymouth took about 35,000 to Wembley [for the play-off final against AFC Wimbledon]. At the training ground hundreds of them waved us off. It's something you obviously don't get at Under-21s football and that's another amazing experience I'll remember forever."