You will have to create a function first, that has your command in it. Then create an alias to that function.
PS C:\Users\jpogran\code\git\scripts> function get-gitstatus { git status }
PS C:\Users\jpogran\code\git\scripts> get-gitstatus
# On branch master
nothing to commit (working directory clean)
PS C:\Users\jpogran\code\git\scripts> Set-Alias -Name gs -Value get-gitstatus
PS C:\Users\jpogran\code\git\scripts> gs
# On branch master
nothing to commit (working directory clean)
You might also be interested in the OS project called posh-git that aims to provide a powershell environment for git commands. Wraps git commands with PS type functions and also provides a new prompt that shows the status and branch in your prompt.
EDIT: Forgot to add how to find out how to do this using Powershell.
PS C:\Users\jpogran\code\git\scripts> get-help set-alias -examples
This will show you examples (the last one applies here) of how to use set-alias to create aliases to commands with paramaters, pipelines, etc.