I'm no expert but am currently tandem-feeding my two DC (8 mo and 2.5 yo).
Some people produce colostrum towards the end of the pregnancy, some don't. There's no harm in trying to express some if you want to, but don't be disappointed if you can't get any. Look on YouTube for videos on hand-expressing if you're not sure about the best way to do it. Fwiw, I had none during DC1's pregnancy but loads during DC2's (which DC1 drank very happily!).
Colostrum can be frozen and saved for baby, I believe, but I've never done it.
It might be worth having a good look at kellymom.com for more breastfeeding info. La Leche League is also a great source. Get your DP and family on board and make sure you have support around you in the early days. Breastfeeding is hard, especially to start with and with a toddler around and it's not helpful if you're being undermined by well-meaning or otherwise relatives.
Also, read tonnes of threads on here! There's a poster called Tiktok who really helped me (unbeknown to her!), because she always gives such good, well-thought-out advice.
Read as much as you can about latching on, supply, how milk is made, blocked ducts, mastitis, demand feeding, sleep, etc, before baby is here, so that you feel more prepared. I don't know how GD affects breastfeeding, but I'm sure you'll be able to do it with the right support.
Good luck!