How to determine gateway address when going from DHCP to static IP address with Ubuntu

"To get your gateway's IP address, use the route command with the -n flag to translate hostnames into IP addresses (or rather, disable DNS lookups on the IP address).

$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.10.10.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 10.10.10.1 0.0.0.0 UG 100 0 0 eth0

The default gateway is the one where the destination is 0.0.0.0, in this case, 10.10.10.1.

Though I'm wondering, why are you manually assigning a static IP? Does your access device (router, wifi, whatever) not allow DHCP reservations? Most devices do these days. You just need to provide the MAC address, and many will already know the MAC by hostname. If your's doesn't you can get the MAC from ip, the ""link/ether"" address:

$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000
link/ether 00:50:56:10.10:21 brd ff:ff:ff:ff:ff:ff
inet 10.10.10.121/24 brd 10.10.10.255 scope global eth0
inet6 fe80::250:56ff:fe10.1021/64 scope link
valid_lft forever preferred_lft forever

or ifconfig, the ""HWaddr"" address:

$ ifconfig
eth0 Link encap:Ethernet HWaddr 00:50:56:10.10:21
inet addr:10.10.10.121 Bcast:10.10.10.255 Mask:255.255.255.0
inet6 addr: fe80::250:56ff:fe10.1021/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:107139 errors:0 dropped:0 overruns:0 frame:0
TX packets:12345 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:29375815 (29.3 MB) TX bytes:1447714 (1.4 MB)
Interrupt:18 Base address:0x2000"

jtimberman and freiheit provided good answers. Let me add one thing: it is not clear if you are the network administrator or not. If you are not, assigning IP addresses statically when the network is supposed to use DHCP is really playing the Sorcerer's Apprentice. You can create a lot of problems (assigning an existing IP address, for instance).