There is nothing in Tums that would affect your GTT (they are chalk) but ask when you get there - I wouldn't take them beforehand. If you are not eating, hopefully you won't have heartburn, though that sickly sugary drink will probably give it to you. It gave it to me. I love my Tums while I am pregnant as well

The pathology lab was probably confused about what your doctor has given you the form for and recommended you eat because they think you are having a GCT rather than a GTT, especially if they haven't seen the form. I would say they are just making an assumption based on routine procedures of it being your first test, but it is not that unusual for doctors to send you straight for a GTT if you are considered high risk.

Good luck with your test tomorrow.