The „border“ was for a good part invented by Caesar to explain why he was unable to form a strong base east of the Rhine and retreated. You have „Gallic“ (i.e. Latene) finds and possibly settlements in southern Germany, northern central Germany and Bohemia. So in any way the tribes claiming to be Germanic (according to the finds living in what is now northern Germany, southern Scandinavia and Poland) were in regular contact with their Gallic neighbors.