That requires some in-depth knowledge about how your garage door opener works, and I suspect the sprinkler zones are not suitable for switching garage doors, at least not without replacing the triac with transistors. The reason is that I assume (this is just a guess) garage door openers operate in DC, and sprinkler valves work in AC. The triac on OpenSprinkler can switch on and off AC voltage but not DC voltage. Of course, if your garage door opener works in AC then it’s likely this can work. If it’s DC, you need to replace the corresponding triac with a NPN transistor (such as 2N3904 for small current, or MPSA14 for high current), or N-Channel MOSFET (such as BS170). Then it can switch DC voltage.
Do note that all current versions of OpenSprinkler have a built-in relay, and it’s included particularly for switching devices like garage door openers. Although the software hasn’t been upgraded to provide built-in support for the relay, it has been tested and is functional. You just have to customize the code a bit to suit your need.