this means that the timing will be 180 degrees off, so now it isn't okay. It will not bend valves but the car will run badly. There is an incredibly detailed timing procedure for this vehicle so taking it off and putting it back on in the same place isn't enough. I will try and copy and paste the procedure for you if the GMM server will allow it:
Important: Ensure that the alignment of the crankshaft is not 180 degrees off. The alignment marks must align with the corresponding notches on the rear timing belt cover.
Important: Do not rotate the crankshaft if the camshaft are not locked in place with 1-2 and 3-4 camshaft locks, J 42069 and J42069-2 .
Important: Do not rotate the camshafts unless the crankshaft is at 60 degrees BTDC or valves may contact crankshaft.
Important: Failure to follow the described timing belt inspection and maintenance may result in a broken timing belt and cause severe engine damage.
Important: With the camshaft locks installed, verify the distance between camshaft sprockets with the green test belt located in the J 42069 .
Tighten Tighten the upper pulley bolt to 40 N·m (30 lb ft).
Important: If TDC is passed, do not rotate crankshaft counterclockwise. This will not allow proper slack to be taken up between the timing belt tensioner and the crankshaft sprocket. To correct, rotate the crankshaft an additional two turns.
Important: Timing marks on the timing belt are used for initial installation only.
Important: If the timing marks are off, repeat the procedures to readjust. Removal of the timing belt is not necessary.
Tighten Tighten the timing belt tensioner fastening nut to 20 N·m (15 lb ft).
Tighten Tighten the battery terminal bolts to 17 N·m (13 lb ft).