The Bayern forward, who scored an all-time league record 41 goals last season, volleyed in from a corner to level in the 42nd minute after Alassane Plea had put the hosts in front in the 10th.
There was no time for new Bayern coach Julian Nagelsmann and the champions, who had won none of their four pre-season warmup matches, to settle in, with Gladbach instantly pressing high and Patrick Herrmann firing narrowly wide in the fourth minute.
With 23,000 fans allowed in the 60,000-capacity and firing up the home team, Herrmann should have finished a quick break himself a little later, instead of miscuing a pass for Lars Stindl.
With Bayern's backline repeatedly caught out of position, Plea did not miss when he latched on to a well-timed through ball from Stindl in the 10th minute to rifle past keeper Manuel Neuer.
Lewandowski, however, moved quickly to beat his marker three minutes from halftime to volley in the equaliser, and he came close adding a second early in the second half but was denied by yet another superb Sommer save.
Gladbach's Marcus Thuram also should have done better when he failed to connect with the ball in front of goal late in the game. He then twice protested for a penalty after going down inside the box following challenges by Bayern defender Dayot Upamecano.