您可以尝试！

"version": "0.1.0"
"command": "g++",
"isShellCommand": true,
"args": [
         "main.cpp",
         "`pkg-config", "--libs", "--cflags", "gtkmm-3.0`",
 ]

您应该控制引号的引用方式。

更多信息： Visual Studio代码中的任务

you should control how the argument is quoted.
more information: Task in Visual Studio Code