I was setting up IFTTT using the 2 pre-made Garadget applets on my Android phone and encountered the same issue as above. Setting up the close garage door was fine but when I went to setup google assistant with the Open garage door, it was looking for me to enter a $ sign as a variable. I know I can create my own IFTTT applet with GA and Garadget but I thought I'd let you know.
PS. Just installed the Garadget and app working great. In the process, I also fixed an intermittent fault with my garage door, win win.
EDIT: OK, figured out the IFTTT official Garadget/Google Assistant setup. I think this is where having multiple doors will be useful for the $ option.
"OK, Google, Open the $ door." Where the $ is the name of the door you have set in the garadget settings.
Google Assistant response "OK, Opening the $ door."