I have a Garmin 500 bundle with Hrm coming in the mail. I am excited to get my phone off my handlebars and especially the he functions to pace my long rides.
My question is can I take the head unit to the gym and use it just for the harm function while using the spin bike or will I need another receiver/phone app to use this at the gym.
I have a galaxy s 6, but I use my old S3 as my gym mp3 and timer. Thanks for your help.

Yes, I use my 500 and the HR strap on spin bikes, rower, etc...you can turn off the GPS while indoors.

Yes. Turn off the GPS and stick it in your pocket.

Awesome, thanks, i knew the garmin could be used with the speed/cadence sensor on a trainer but wasnt sure if it would work with just the HRM on other equipment.

It won't talk to the gym bike, so if your gym has stationary bike that report power or whatever, you're just going to get time and heart rate. But that will work, and the calories it reports should be pretty close to the truth.

I use one of the spin bikes at the gym so no electronics to communicate with, just intrested in heart rate for intervals/workload. The only two options at my gym are one of the two spin bikes or two recumbent bikes. Since i dont get speed/distance data on the spin bikes, I am hoping the HRM will help me get the most out of my workouts when weather forces me inside.

Time and effort are all that really matter so your plan is sound.

^^^+1. ...and maybe cadence.