typoscript condition on get parameter

Question

I'm trying to include separate setup.ts files, based on a "custom" GET parameter, but no matter I give ?custom=1 or not, the code always enters the ELSE clause and I get "bbb" on top of the page. Where am I wrong?

[globalVar = GP:custom = 1]
<INCLUDE_TYPOSCRIPT: source="FILE:fileadmin/templates/typoscript/CUSTOM/setup.ts">
page.1 = TEXT
page.1.value = aaa
[else]
<INCLUDE_TYPOSCRIPT: source="FILE:fileadmin/templates/typoscript/setup.ts"> 
page.1 = TEXT
page.1.value = bbb
[end]

Answer 1

It looks like it's impossible for included files:

https://forge.typo3.org/issues/29583