Shadowing means observing dentists at their trade. Most schools have a recommended or required amount of hours to observe dentists or at a dental related field (like a free dental clinic provided by the community or organization). Usually, your expected to get in at LEAST 100 hours of observation time before interview.
As far as your GPA, the school i'm applying to doesn't care if you have a 25 or a 21 on the DAT. Because the school is on a admission point system, the same amount of point is rewarded for those who have 21 and above. Meaning, on a 5pt scale, if you have a 21 DAT or above you'll get 5 points for the DAT portion (5 points is the highest you can get regardless if you make a 25 on DAT). If you get 20-19, you get a 4 point for the DAT portion. For your GPA, if you have 4.0-3.8 gpa, you'll get the total 5 points for the GPA portion, If you have 3.7-3.5 you'll get 4 points...and so on...But keep in mind, not all schools are the same.
Also, whenever you explain why your GPA is not as competitive as other applicants, make sure you have a good explaination - they want to know you have matured from your experience and that you are a responsible adult.
That's the best advice i can give you. Best of luck.