As already answered by other you need a gate voltage above the source. Depending on the mosfet type it could be 5 to up 15V (usually, the more, the better, unless you break the Vgs maximum). Since you are driving in continuous your losses will be almost only from Rds, so a large gate drive will help (there are curves or specs for that in your mosfet datasheet). These days mosfets are usually characterised at least 4.5V and 10V.
Now, for driving the gate. You don't need a big current even if it's a big gate since you are essentially making a load switch. In fact a small gate current helps softening the load transient turning on the mosfet gradually.
Most commercial gate drivers are designed for PWM: they have large current capabilities to reduce switching losses and they generate the gate voltage by bootstrap (a kind of charge pump). In short, they don't work in high side without at least some turn off interval (specified in the driver datasheet).
For switching your 48V load you'll need something like 55-58V (or something like 10V referred to the source). The first option is simply using an high side gate driver designed for 100% duty cycle. They have an embedded charge pump, you solder them in and it's all done. Another option (mentioned in another answer) is a photovoltaic gate driver. They are somewhat expensive but work well; they also give you galvanic insulation which is handy with IGBTs, for example.
If you have many of these load switches you could consider to generate your own gate voltage with a converter. A boost of 25% for some mA is really easy to do. Maybe you can also tap some similar voltage from somewhere else. In any case remember to protect the gates with zeners if there is a risk of running over the Vgs maximum, especially since probably your 48V battery can presumably go low (to 36V, maybe? depends on the application)