Escape the period character in a command flag within a PowerShell script?

Use the & invocation operator to invoke the program and use regular ' single-quotes to qualify the argument boundaries:

& gfpca '-run.config' 'default.conf'

Now, PowerShell will pass -run.config and default.conf to gfpca as-is without attempting to interpret them as native parameters

tl;dr

Due to an unfortunate bug, present up to at least PowerShell 7.2.1, an argument passed to an external program that starts with - and contains . requires escaping or quoting, which in the simplest case means:

# Note the ` before -
gfpca `-run.config default.conf

To be clear: This shouldn't be necessary.

Mathias R. Jessen's answer provides an effective solution, but let me add some context:

• You only need to escape or quote one of your arguments, -run.config, due to a PowerShell bug, where passing an unquoted argument that starts with - and contains a . to an external program is mistakenly passed as two arguments.

  • In other words: Without escaping or quoting such an argument (see below), PowerShell mistakenly passes -run.config as separate arguments -run and .config

  • The bug, present up to at least PowerShell 7.2.1, has been reported in GitHub issue #6291, where it was acknowledged as such, but is awaiting a fix; it is one of several bugs relating to --prefixed tokens.

• While using &, the call operator, unconditionally for command invocations is a robust approach, it is important to note that it is only necessary if the command name or path is quoted and/or contains variable references - which isn't the case here.

  • See this answer for details.

• Invoke-Expression (iex) should generally be avoided - see this answer. Specifically, do not use it to invoke an external program or PowerShell script - see this answer.

  • As an aside: Your call, InvokeExpression('...'), uses pseudo method syntax, which is to be avoided: PowerShell functions, cmdlets, scripts, and external programs must be invoked like shell commands - Invoke-Expression '...' - not like C# methods. That is, don't put ((...) around the argument list, and separate argument with whitespace, not with ,. See this answer for more information.

While ` (PowerShell's escape character), which you tried to use, is (a) key to the solution, it must be applied to the initial -, which prevents PowerShell from (inappropriately) parsing the argument according to the rules of PowerShell-native parameter names.^[1]

# Note the ` before - 
gfpca `-run.config default.conf

As shown in Mathias' answer, using a quoted string around the whole argument (e.g. '-run.config') is an alternative.

^{[1] Because parameter names aren't expected to require escaping, what follows - is parsed verbatim, up to the first .. Therefore, your attempt to `-escape the . character was ineffective: ` wasn't recognized as an escape character and therefore retained, and the bug still kicked in. By contrast, `-escaping the initial - prevents PowerShell from parsing the entire token as a parameter, which bypasses the bug}