Those votes that favor Gary Johnson as a Libertarian candidate will favor Trump because it means less votes for Hillary Clinton. Those voters who choose the Libertarian party would likely have aligned with the Democrats rather that Republican candidate. So Trump does not have a negative impact by encouraging voters to select Gary Johnson over Hillary.
He could tilt the election towards trump. If he pulls people who would otherwise vote for a democrat or Hillary Clinton he has the potential of tilting the election towards trump. However, if he pulls republicans it could go the other way instead. Either way, he could tilt the election.
Gary Johnson has become a popular new candidate for people who are either "Never Trump" or "Never Hillary." He is a Libertarian who believes that getting exposure for his party will "move the needle" in his direction. This belief means that he will not encourage more people to vote for Trump - they will instead vote for Johnson!
Gary Johnson, running on the Libertarian ticket, will not tilt the election to Donald Trump. He is a third party candidate, and this traditionally has received only a minimal amount of the vote. Given the high stakes of the election, most people will vote for one of the two major candidates.