The 24-year-old was released by the Toffees earlier this summer after scoring just three goals in 39 appearances during his time at Goodison Park.
However, Lions boss Ian Holloway has given Gueye the opportunity to remain in England by handing the Frenchman a 12-month deal at The New Den.
The former Strasbourg attacker becomes Millwall's fifth signing of the summer.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Sunderland | 9 | 6 | 1 | 2 | 18 | 8 | 10 | 19 |
| 2 | Sheffield UnitedSheff Utd | 9 | 6 | 3 | 0 | 12 | 3 | 9 | 19 |
| 3 | Burnley | 9 | 5 | 3 | 1 | 14 | 4 | 10 | 18 |
| 4 | West Bromwich AlbionWest Brom | 9 | 5 | 2 | 2 | 12 | 6 | 6 | 17 |
| 5 | Leeds UnitedLeeds | 9 | 4 | 4 | 1 | 15 | 7 | 8 | 16 |
| 6 | Watford | 9 | 5 | 1 | 3 | 14 | 14 | 0 | 16 |
| 7 | Norwich CityNorwich | 9 | 4 | 3 | 2 | 16 | 10 | 6 | 15 |
| 8 | Blackburn RoversBlackburn | 9 | 4 | 3 | 2 | 15 | 11 | 4 | 15 |
| 9 | Middlesbrough | 9 | 4 | 2 | 3 | 10 | 7 | 3 | 14 |
| 10 | Oxford UnitedOxford Utd | 9 | 3 | 3 | 3 | 13 | 11 | 2 | 12 |
| 11 | Swansea CitySwansea | 9 | 3 | 3 | 3 | 8 | 6 | 2 | 12 |
| 12 | Derby CountyDerby | 9 | 4 | 0 | 5 | 12 | 12 | 0 | 12 |
| 13 | Hull City | 9 | 3 | 3 | 3 | 12 | 13 | -1 | 12 |
| 14 | Plymouth ArgylePlymouth | 9 | 3 | 2 | 4 | 10 | 13 | -3 | 11 |
| 15 | Sheffield WednesdaySheff Weds | 9 | 3 | 2 | 4 | 11 | 15 | -4 | 11 |
| 16 | Stoke CityStoke | 9 | 3 | 1 | 5 | 10 | 12 | -2 | 10 |
| 17 | Bristol City | 8 | 2 | 4 | 2 | 9 | 13 | -4 | 10 |
| 18 | Millwall | 9 | 2 | 3 | 4 | 12 | 11 | 1 | 9 |
| 19 | Preston North EndPreston | 9 | 2 | 3 | 4 | 7 | 12 | -5 | 9 |
| 20 | Coventry CityCoventry | 9 | 2 | 2 | 5 | 10 | 13 | -3 | 8 |
| 21 | Luton TownLuton | 9 | 2 | 2 | 5 | 8 | 15 | -7 | 8 |
| 22 | Queens Park RangersQPR | 9 | 1 | 4 | 4 | 9 | 16 | -7 | 7 |
| 23 | Portsmouth | 9 | 0 | 5 | 4 | 9 | 20 | -11 | 5 |
| 24 | Cardiff CityCardiff | 8 | 1 | 1 | 6 | 3 | 17 | -14 | 4 |
| > Championship Full Table | |||||||||
