The 28-year-old, whose previous deal was due to expire in the summer, will now remain at the Priestfield Stadium until the summer of 2017.
McDonald, who is in his third spell with the Gills, has scored 16 goals in 38 appearances this season.
---
Port Vale have re-signed Cardiff City midfielder Tommy O'Sullivan on loan until mid-April.
The 20-year-old was on loan at Vale Park earlier this season, making his only Valiants appearance from the bench against Walsall in mid-February.
---
Doncaster Rovers have signed Rotherham United striker Jonson Clarke-Harris on a youth loan.
The 20-year-old joined the Millers from Oldham in September for a club record fee and has scored three goals in 14 league games for the Championship side.
---
Charlton Athletic midfielder Oliver Muldoon has signed a new contract with the Championship club and joined Gillingham on loan.
The 20-year-old, who is yet to make his senior debut for the Addicks, will remain at The Valley until the summer of 2017.
He will be with the Gills until the end of the season.
---
Leyton Orient will be without Romain Vincelot for the remainder of their relegation battle after the midfielder suffered a knee injury.
The 29-year-old Frenchman damaged medial ligaments in the 1-0 defeat at Peterborough on Saturday and will be sidelined for the rest of the campaign.
Vincelot has scored four goals in 33 appearances for the O's this season.
---
Peterborough United owner Darragh MacAnthony intends to let caretaker manager Dave Robertson and his team continue in the role.
"I haven't made a call and I don't intend to," MacAnthony told BBC Radio Cambridgeshire. "Why would I?
"The three lads who are now in charge, with the rest of the management staff, are doing a great job, and I don't want to put too much pressure on them."
---
Aston Villa and Leeds United are said to be keen on signing Jake Andrews from Bristol City.
A clutch of teams are supposedly hopeful of signing the 17-year-old, with the likes of West Bromwich Albion, Hull City and Ipswich Town also said to be keeping a close eye on the central-midfielder.