There was around $400ish fine for "traveling abroad without the GC by a US permanent resident." The guy at the Europe airport told me to be ready to pay that fine once I land in the US. However, I was not asked for any fee at the US airport. So, I guess, the fine depends on the customs officer's discretion. I was taken in to the secondary screening for additional questions at the airport. They were mainly asking questions as if I was an asylee who has "illegally gone to Nepal and returned from Nepal." When they figured that was not the case, they let me go with information about how to get a replacement GC. I did not have to do that, because my card was later found at the Europe airport and mailed to my address.