Ip can ping with source but not without source

i was doing PBR lab ( http://racks.nwkings.com:8087/legacy/ROUTING/ROUTING/PBR.unl/topology )
here i configured ip address and default routes on gateway router, i checked if pc1 and pc2 can reach isp1 and isp2.
it was not reachable so i configured route towards gateway on both isp1 and isp2.
now i configured nat on isp 1 and isp2 then checked if i can ping 8.8.8.8 so i was unable to ping and when i took source 1.1.1.1 its was pinging
can i please explain whats the issue

ateway of last resort is 2.2.2.2 to network 0.0.0.0

 1.0.0.0/30 is subnetted, 1 subnets

C 1.1.1.0 is directly connected, FastEthernet0/0
2.0.0.0/30 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, FastEthernet0/1
C 192.168.1.0/24 is directly connected, FastEthernet1/0
S* 0.0.0.0/0 [1/0] via 2.2.2.2
[1/0] via 1.1.1.2

GATEWAY#ping 8.8.8.8 so 1.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
.!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 36/42/48 ms
GATEWAY#ping 8.8.8.8

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:

Success rate is 0 percent (0/5)

Gateway Physical IP subnet is there in the ISP Router ?