Keep it separately, but do remember to take it with you when you leave the country. Make photo copies and keep them with you (in case you loose the original card)
If you go outside the country with out this, you will have to have someone send it to you in order for you to re-enter the country. Although the officer at the POE (if you can get this far - Airlines will not transport you to POE without proof of GC) may let you in if he is in a good mood to verify your record based on your A#.