Many players ask the question about healing units, there are 2 kinds of units, All units heal at day change without using Gold for instant actions.
Organic units > have morale and consist of Infantry and Calvary, they heal at 1/7th (14.28%) the difference in current morale and the province's morale which they are located , they must be in home province.
Mechanical units > have condition and all other units are considered as mechanical units, they heal at 1/7th (14.28%) the difference in current condition and 100%, only planes must be in/using home province (airfield) to heal.
Please NOTE> that organic units gain morale after winning a battle (even if they are stacked with range units). there is third way for a unit to heal (info is present in the S1914 GAME INFO post below): in foreign terrain units below 50% restore to 50% (1/10th the difference) Organic units also decrease morale because of territory since not all organic units will heal if the provinces morale is lower than the organic units morale, these same units lose morale while at sea (-5% per day / leveling off at 50% morale) if they are not escorted (stacked) with a naval ship at sea.
Please reference > S1914 GAME INFO