Pull EN pin of regulator to ground
A better solution would be to use a PNP transistor to control the EN pin. A PNP is ON when the input to it is LOW (or lower than the collector minus 0.7V, that is).

Take this circuit, for instance:







simulate this circuit &ndash; Schematic created using CircuitLab

When the GPIO is HIGH the transistor Q1 is turned off (GPIO >= VCC-0.7). Also if the GPIO pin is disconnected it is turned off because R3 pulls it up to VCC.

Set the GPIO LOW and Q1 turns on since the base is now lower than VCC-0.7V. With Q1 turned on it connects the EN pin to ground, so it becomes LOW.

If you replace the PNP with an NPN (there's more chance you have one in your bits box) you can basically invert the logic:







simulate this circuit

Now when the GPIO is LOW or pulled down by R3 Q1 is off and EN is pulled HIGH. Set the GPIO HIGH and Q1 turns on and connects EN to ground setting it LOW.

In both these scenarios the EN pin defaults to HIGH. If you want it to default to LOW until you activate the GPIO pin (i.e., during bootloader operation when the GPIO is in INPUT mode) you can reverse R3 so it becomes a pull-down (for the PNP) or a pull-up (for the NPN).