I'm getting ready to install a Gil sea strainer but I have a few questions. Should I install this before or after the sea pump. Also which is the inlet and which is the outlet (upper/lower holes)? Thanks

Before the pump and in is high and out is low.

What he said.
You also want to keep the stainer fairly low in the boat so it is not hard to prime. If it's below the water level, then you may want to install a shut off valve (ball valve) before the strainer. This is so that you can shut off the water if you need to clean the strainer while in the water. If not, water will pour in the entire you are cleaning it. Good luck. Let us know if you have any more questions.
Eddie

Thanks!