Hi Saulex,
You can use the following:
<util:available uri="file:${fullpath}">
....
</util:available>
With fullpath being the full path where your file is "supposed" to be stored.
If it is available, then the process will run the code where the "..." is.
Make sure to declare: xmlns:util="jelly:util" in your <gel:script .... >
Hope that helps..