Replacing Citrix Secure Gateway with LTM/APM?

Yes, you most certainly can proxy ICA traffic the same way as CSG does, and if you don't want to do any authentication on APM, that is fine as well you can just run through the iApp and put in bogus Active Directory information in there, and then go the Access Policy and remove the AD Auth object from it - and that should be it. You still need to have APM licensed, as ICA proxy functionality is sold and supported only with the APM license.