Sam Winnall gave Burton a first-half lead, but a Gillingham fightback was rewarded when McDonald grabbed his 20th of the campaign.
Defender Matthew Lawrence had a header cleared off the line as the Gills threatened to take control - only for the visitors to be rocked in the 27th minute.
Winnall got a firm header to Andy Corbett's cross and ball found its way inside the far post despite goalkeeper Alan Julian getting a hand to it.
Gillingham's pressure paid off when McDonald levelled the scores with a neat turn and 12-yard shot in the 78th minute.
Source: PA
Source: PA