The reason is because there is writing, but if there is no writing; no - טעמא דאיכא כתב הא ליכא כתב לא
Summary:
The גמרא assumed that שמואל maintains that it is a גט even if later he said it was just a נייר חלק. In the הו"א the גמרא assumed that she is מגורשת ודאי either because the בעל is always נאמן , or because it was actually read before they gave it to her, or because they viewed the גט from afar.
And שמואל also said, we are concerned - ושמואל נמי חיישינן קאמר
Summary:
If one gives his wife a נייר חלק and says הרי זה גיטך there is a חשש גירושין, even if we did not examine it במיא דנרא. If however it was examined and no letters showed up, there is no חשש of גירושין.
Summary:
The גט should be read by the ע"מ before the נתינה (and preferably after the נתינה), otherwise she may not remarry (unless it is read after the נתינה). [If the גט was not read at all there is a dispute whether אם נשאת תצא or not.]
