I have a IRF4905 in a trailer brake controller and a L1404s in a mighty mule gate opener. I have some miscellaneous mosfets. What is the important characteristic to look for in possible subs. Vdss and Id need to be equal or greater and Rds equal or less. What about Vgs(th)?

It depends what the drive signal is. If your drive logic is powered from 5V, you need a logic-level FET that turns ON well with 4.5V of gate drive. If your drive circuits swing to 10 or 12V, you don't need logic level FETs but you can use them if you want.