What tone is 杆 in 扶杆？
Let's talk in terms of Traditional Chinese here, which I feel is going to be much clearer.


「杆」is not pronounced gǎn. The valid pronunciations listed in dictionaries are gàn and gān.
「杆」pronounced gān means pole or rod.
「桿」is only pronounced gǎn, and is a synonym of「杆」when it is pronounced gān, in the sense that「桿」can mean pole or rod.
「桿」(gǎn) also means lever or handle.


This is not hard to remember if you consider that「干」is never pronounced in the third tone, and the other common character which uses「旱」as a phonetic component,「趕」(gǎn), is also pronounced in the third tone.

Simplified Chinese merged「杆」and「桿」into one character.





The「杆」mentioned in the text is the horizontal bar referenced in the image above (扶手橫桿/扶手横杆), which should be pronounced gǎn. From Panlex:


Searching héng gǎn or hénggǎn  results in 横杆 with definitions cross-bar, rail, overarm
Searching  héng gān turns up no results
Searching  hénggān results in 横竿


(You can check Panlex's dictionary sources here by filtering with the key 'cmn'.)