Johnson has made his loan move to the Gills permanent after originally joining on a temporary basis from Southampton on January 8.
There has been no confirmation on the length of the contract, but his deal at Southampton was up at the end of the season and it is believed he now has the rest of the campaign to fight for a contract at Gillingham beyond this season.
Johnson made his Gillingham debut the day after signing for the club, playing 90 minutes in a 1-0 defeat to Burton Albion on January 9.
He looked impressive in spells and was one shining light in a poor Gills performance, showing desire to run direct from the left of midfield and track back when required.
With Trae Coyle returning to Arsenal from his loan stay with the Gills, alongside Zech Medley, Johnson provides another wide option and making his deal permanent will give Gillingham more flexibility in the loan market - five loanees can be named in a match day squad.
Southampton duo Tom O’Connor and Callum Slattery, alongside latest addition Olly Lee, are all at Gillingham on a temporary basis. Realistically, manager Steve Evans can now add two or three more to the books.