If you want to use CSG, you have to set CSG in order to proxy ICA from public access (I don't remember the gateway mode to use). That's means CSG has to know the public FQDN and patch ICA for that. CSG reads HTTP headers in order to know that. You can keep LTM in front of CSG.
With APM, APM will connect directly to Citrix infrastructure and patch the ICA for you. CSG is not mandatory in that case. There is only one check box to check in Virtual Server.
Please contact your local F5 representative (F5 staff or partner) in order to be advices locally.
Regards,
Matt