Re: EX4200/EX4500 does not pass via the GRE traffic going from devices connected to the EX.

You know, in this situation you had to get more hops during performing a traceroute from PC1 to PC2. *If PC1 has default gateway and if no additional filtering/firewall. It seems like GRE is not used.
Try to use Steve advice. But, I'm not sure that is a solution. Static route with the gr interface as a next-hop is implemented for /32 network mask config. In your situation EX will be know about outgoing interface for 10.255.111.10 and it will be a gr interface. Or may be I'm wrong and junioper has its own behavior for gre tunneling.

These are working SOFTWARE-BASED GRE configurations from my EX8200 and EX4550:

set interfaces gre unit 0 tunnel source 192.168.120.220
set interfaces gre unit 0 tunnel destination 192.168.120.221
set interfaces gre unit 0 family inet address 5.5.5.2/24

root56@j8208# run show route table inet.0 5.5.5.0    

inet.0: 485 destinations, 486 routes (485 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

5.5.5.0/24         *[Direct/0] 00:22:56
                    > via gre.0

root56@j8208# run ping 5.5.5.1 
PING 5.5.5.1 (5.5.5.1): 56 data bytes
64 bytes from 5.5.5.1: icmp_seq=0 ttl=64 time=0.318 ms
64 bytes from 5.5.5.1: icmp_seq=1 ttl=64 time=0.125 ms
^C
--- 5.5.5.1 ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.125/0.222/0.318/0.097 ms

Try to replace gr-0/0/0 by gre