It seems this is possible because this is a melee map and therefore there are no triggers (apart from the standard initialisation which you can just copy from any old map).
As a courtesy, please try very, very hard to contact the mapmaker first and get the map properly.
However, it does seem rather ridiculous that we don't have GSL maps, and unfortunately they all seem to be locked on NA now, so the following is an outline on how to get such a map across. Please don't just go and copy any old map, this is really just intended so we have proper tournament maps on SEA.
The steps below aren't perfect (some of the art, in particular the big GSL decal seems not to survive) but I was able to play it and it seems to work.
Rough, unrefined steps (only tested with Abyssal City, YMMV):
Open the StarCraft II Editor and save a blank map.
Open the blank map with a MPQ editor (recommend Ladik's editor; yes, my love affair with WinMPQ has ended).
Extract the ComponentList.SC2Components file from the blank map.
In a text editor, remove the Triggers line and replace with <DataComponent Type="plob">Objects</DataComponent>
Create a game on NA with the map you are interested in so it gets downloaded to your %ProgramData%\Blizzard Entertainment\Battle.net\Cache and grab the .s2ma file (should be on the order of megabytes in size). (To make it easier to find you can delete everything from there first so when you sort by date the file will be among the last modified.)
Copy the .s2ma file from cache and rename it from .s2ma to .sc2map.
Open it in the MPQ editor and add the edited ComponentList file.
In the StarCraft II Editor, open the blank map and open the Modules -> Triggers window. Copy the existing trigger (Melee Initialization).
Open the downloaded map in the StarCraft II editor and go to the Triggers editor. Paste the previously copied trigger.
Go to Map -> Map Options. Under Publishing Options untick Arcade Map and tick Automatically Add Multiplayer Data.
Go to Map -> Game Variants and tick Use Default Variants (at the bottom left).
Go to File -> Dependencies and remove Liberty Multi (Mod).
At this point you should be ready to publish the map. Check you are connecting to the right server (File -> Preferences -> Battle.net -> Region to Oceanic) and then publish it.
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