The tape is not going to lie, and what it tells Dave Gettleman is what is going to determine what happens next with Eli Manning. During his introductory press conference last Friday, the new Giants general manager said he would move forward with Manning unless what he saw in the Week 15 loss to the Eagles was “a mirage.’’ Sure enough, that was, by far, the most productive game of the sad season for Manning, and one or two strong grades amid a bunch of failing marks is not going to cut it, not even for someone with Manning’s pedigree. And so, there are no promises Manning, who turns 37 Wednesday, will return for a 15th season with the Giants. Not until Gettleman studies what really went down with the franchise quarterback in 2017. “I’ve got to sit down and study tape,’’ Gettleman said Tuesday on WFAN. “I’m gonna study the tape on Eli, I’m gonna study it chronologically. I only was able to watch a couple of Giants games front to back and one of ‘em happened to be the second Philly game. The guy that I saw that day was a guy that helped put two rings on my fingers.’’ As he pours over game after game, Gettleman is sure to see some unsightly Manning moments as he operated behind an ever-changing and usually underperforming offensive line, often had little or no ground game to balance out the attack and frequently had backup or practice squad-type receivers as targets. There was also plenty of substandard play from Manning, and Gettleman is going to have to sort out where to parcel out the blame for the offensive contributions to a 3-13 season.