I think they do the glucose test for almost all pregnancies, to be cautious.
If you do have GD, its important to know so you can treat it. Even being overweight, they never tried to fudge the results or anything for me... so I never had to deal with the "false treatment just in case" - but I did get tested for it every time.
GD definitely isn't a result of *just* diet or weight... it can be something that "just happens" with some pregnancies, even if you're a 120 lb nutritionist, kwim? Wouldn't hurt to check it out.
Being that you (we all) started out overweight with the pregnancy, its not abnormal to gain a small amount. With my second baby I only weighed 7 lbs over my pre-pregnancy weight when she was born. I had lost 8 lbs the first trimester, and then gained that back, plus seven for a total of 15 lb gain during my pregnancy with her. She was born 2 days before her due date, at 7lbs 11oz, very healthy. With this pregnancy I've already lost 3 lbs (and I'm 7 weeks along today)
If you do start gaining weight from here on out, don't worry about that either. Its normal.
With my first I gained 35 almost 40. My mom gained 65 with my brother. It happens.
As long as you're eating well, trying to get a little bit of exercise (walking, yoga, etc), and drinking plenty of fluids, you're doing exactly what you need to be.