Fixing a Pergo separation
There is generally a lot of friction, making adjustment difficult, so you might have to either disassemble and redo it, or just fill the gap with the matching repair putty.  It will be noticeable because it will be a solid color, but less noticeable than a gap.  

However, it doesn't hurt to try to close the gap.  At the end of that row farthest from the gap, you will need to keep that row and both adjacent rows from moving. Take some scrap pieces of laminate (or wood of the same width as the planks), and cut a set of filler strips to match the gaps between each of the three rows and the wall.  Wedge those in place temporarily just for the adjustment.  

On the other end of the problem row (the end closest to the gap), you will be trying to push or pull the end of the row to close the gap.  There is a tool for that.  It is a bar or heavy strip maybe 18" long, with a lip at each end.  You lay the bar on the end of the row.  One lip goes down to capture the end of the row.  The other lip sticks up and you tap it with a hammer to pull the gap closed.

If you don't have that tool, you can use a wide pry bar as a lever.  Put the tip of the bar against the end of the problem row.  Above that, stick a long block of wood between the bar and the wall to act as a pivot.  Tap the bar with a hammer above the pivot and in the direction of the wall so that the tip of the bar is being pushed against the end of the plank.  

With either method, don't tap so hard that you damage the end of the plank or dent the wall.  A lot of light taps can be as effective as a heavy whack and less likely to damage anything.

The planks might not move at all, or may make the gap narrower but not completely tighten it (or it may close and then rebound open so that you can't totally close it).  But if it doesn't close the gap, any improvement you get will make filling less noticeable if you decide to go that route.  

Just some perspective, though, if you can't push the row tight--the flooring will be there for a very long time, and you might kick yourself later for filling the gap rather than doing it right.  If you still don't have many rows installed beyond that, consider biting the bullet and redoing that part of the row while the task is still not insurmountable.