I had this exact experience a few weeks ago with Air Canada out of Vancouver. I ended up with the dreaded GTE on my boarding pass, buy essentially refusing to be extorted ( paying for preferred seating)
You would think, buying the damn ticket entitles you to a seat, but not in Air Canada, unless you are willing to pay extra.
the most recent time I had this issue, The office travel coordinator booked my flight, I got an email asking me to go online and select a seat. except the system wanted to charge me $15 to select a preferred seat, and no option to select a seat without paying extra.
I actually called customer support and they told me, the only way you can be guaranteed a seat, without paying extra for it is to wait and login to their website exactly 24 hours before the flight and manually choose a seat.
otherwise you will not be guaranteed a seat and will be on standby and assigned a seat at the gate.
It felt like extortion, and is one of the sleaziest companies I have ever dealt with.
unfortunately, I cannot book my own business trips, and it takes a while for the new travel coordinators to believe me when I tell them "I would rather crawl across the country then fly on Air Canada"