Didn't need branch in the end at all

workflow/playpen.smk

@@ -8,14 +8,8 @@ rule a2b:
     output: "{path}.b"
     shell: "sed -e 's/a/b/g' {wildcards.path}.a >{wildcards.path}.b"
 
-def wildcard_path_endswith_gz(wildcards):
-    return wildcards["path"].endswith(".gz")
-
 rule gunzip:
-    input:
-        branch(wildcard_path_endswith_gz,
-               then=[],
-               otherwise="{path}.gz")
+    input: "{path}.gz"
     output: "{path}"
     shell: "gunzip -k {wildcards.path}.gz"