You can't gateway through 10.10.15.100 unless you have a route to it.
Code:
route add -net 10.10.15.0/24 eth2
route add -host 10.10.15.20 gw 10.10.15.100
route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.10.15.20 10.10.15.100 255.255.255.255 UGH 0 0 0 eth2
10.10.15.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2
192.168.78.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2
1.1.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
0.0.0.0 192.168.78.1 0.0.0.0 UG 0 0 0 eth2