global point of view : GateIn is based on JBoss, so it is much more general than the SAP portal.
It is better for any use case (general and wide type of use case), with less specific thing made specifically for SAP.
SAP portal is made for use with the SAP system.
GateIn is more open than SAP.
after that said, ... both are excellent portals.
about your specific questions :
- Look and Feel - Is it easy to create our own stylesheets and generate custom HTML?
But be aware that some PHP plateforms (there are many of them), and some CMS tools/plateforms (there are many of them too), are most of the time much more simple and easy for "building quilckly some web site, some html pages etc..." than a portal platform.
You don't need a portal platform to build some portals (the concept of it).
Some brands like amazon do not use a Portal platform for their portals.
Using a Portal and specifically a J2EE Portal with the JSR286 is not really the simplest tool.
It is robust, made for big project for big portals, with a reliable JBoss AS under for building and running a solid system.
But certainly not the most easy tool for making your HTML pages.
- Application Development and Integration - Is there any kind of limitation in the application development or integration?
None at all.
That's the advatage of J2EE (big) platforms.
- Communication with Web Services - Is there any limitation when using or exposing web services?
None at all.
That's the advatage of J2EE platforms.
Hope it helps for your choice.