To send ETH from Wombat, go to the Wallet screen and find ETH in there. Tap on the Send button, enter the receiver address, the tokens you want to transfer and the amount.
In the pop-up window, check the fees, the gas limit and the nonce. It is advisable to keep the recommended amounts as they are. However, you can set up a higher gas limit to ensure the transaction goes through — any excess Gwei (or nanoeth) will be credited back to you.