Is there a specific Gauge I need for a vacuum pump?

For infusion you should definitely have a gauge - and a digital one is totally worth it if you are doing this professionally or have expensive materials under your bag.  I have one like the GDH200-14 Hojo recommends and it is awesome!  You just get much better visibility on the process with a digital gauge.

For bagged wet layup it is nice but a cheap one will be plenty - you really just need to be sure you're not pulling too much vacuum!  Having a dial gauge on a pressure pot is nice but it doesn't need to be fancy.

Remember that if your gauge is plumbed up next to the pump it will be telling a different story than if it is connected independently to the bag through a separate penetration!