Some of the GPX representatives insist that travel has to be completed by two years from the start date. Others say it has to start by two years from the start date. Some of them have allowed me to extend a couple of weeks without charge because I was close and had a previously extended expiring week. So a lot depends on who you are talking to. I have found GPX to be very understanding though, a couple of times when I had to cancel reservations. All in all I have had good luck with them but I find that the availability at resorts I am interested in seems to be decreasing.
You can find out the use year of your deposit by going to your profile. Click Member Dashboard and select View Profile. Then select Weeks Deposited. The use year is listed as well as the status of the deposits. So I can tell that I have deposited 6 weeks total and used them all. If I hadn't used the 2020 week though, knowing it was a week 10, I would be able to use it to select an exchange week through week 10 (or maybe week 9 based on the comment above) of 2022. I would also be able to pay to extend it for one year only. I only have one week so it's pretty easy. If you have multiple weeks that you can deposit with GPX, you may need to call them to find out which is which. There is a unique reference number for each deposit so they should be able to figure out what was deposited.
GPX also charges to upgrade the number of bedrooms, even if the resort only has two bedroom units if you are using a one bedroom deposit you have to pay. This may be typical of other exchange companies, but other than GPX I use RCI TPUs.
I hope this info is useful. It may be more than was asked...