The phrase "advance garrison" is correct and usable in written English.
It can be used in military contexts to refer to a forward position or unit that is established to provide support or security in advance of the main forces.
Example: "The troops were ordered to set up an advance garrison to secure the area before the main battalion arrived."
Alternatives: "forward outpost" or "preliminary base".
Their foes, the Kuomintang, loomed in the distance, advancing on the garrison town of Yan'an.
Keating took command of the vanguard and advanced on Fort Malartic, the garrison retreating before his troops and blowing up the fort as they left.
It seems likely that Urbicus planned his campaign of attack from Corbridge, advancing north and leaving garrison forts at High Rochester in Northumberland and possibly also at Trimontium as he struck towards the Firth of Forth.
But there was every expectation that at first light, the British would resume their advance and disarm the defending garrison.
Meanwhile the Auckland and Canterbury Mounted Rifles Regiments, supported by the 1st Battalion, British West Indies Regiment, advanced to attack the Ottoman garrison holding Jisr ed Damieh.
While the army advanced towards the city the royalist garrison set alight the suburbs.
The 3rd Light Horse Brigade remained in the area to garrison Jenin until they advanced to capture Tiberias on 25 September 1918, before participating in the pursuit to Damascus.
Italian forces entered British Somaliland from Abyssinia on 4 August 1940, overcame the garrison at Hargeisa, and advanced north-east towards the capital Berbera.
In an act that broke the long-standing stalemate on the island, Ibrahim and his followers advanced on Taormina, defeated the Byzantine garrison before its walls and laid siege to it.
Leaving detachments to hold the captured bridge and fords, Chaytor's Force began their eastwards advance by attacking and capturing the Fourth Army garrison at Shunet Nimrin on their way to capture Es Salt for a third time.
