In most cases the PTT on the radio is simply switching the PTT line to the radios ground line (usually shared with the audio ground). It simply a matter of replacing that switch with the with the output of the optocoupler on the EasyDigi as long as you have low current draw. If you have a higher current draw or some other configuration you can use some kind of simple transistor and/or relay circuit between the EasyDigi and TX PTT.

As far as Legality goes (US/FCC) and as far as I am aware…you will need to tread lightly. Sure, anything goes in a true emergency situations, but what good is it if you are not able to legally practice during non-emergency times. FRS doesn’t allow repeaters and GMRS does, but GMRS requires user radios that have the GPRS repeater channels/pairs and you should have a repeater transceiver and radios that are FCC type accepted for GMRS. GMRS also requires licensing.

If you truly want to do something GMRS for legit purposes for emergency situations, I would probably recommend a basic repeater controller that is a little more “dumb”. This has an operating system and a lot more complexity and might bite you in the butt when SHTF.

I am no GMRS expert,but that is what I understand of the service. Hams have a lot more flexibility on the type of equipment they can use on their bands. Hope that helps.