Is the Guitar Boost feature available without using the Switch-3? I can't find the parameter for it.
No, unfortunately the Guitar Boost feature only functions when you have a Switch-3 hooked up to your Play Electric.
After connecting the Switch-3, you must then assign a button to the Guitar Boost. This is done under the Setup menu on the page entitled SWITCH-3 7/8.
Once a Switch-3 is attached and a button is assigned to Guitar Boost, you will see a "GB" show up in the bottom left-hand corner of the display when the feature is switched ON.
Switching the Guitar Boost ON on the Switch-3 will also cause a change within the Guitar FX menu on the Guitar Amp page 1/5. Namely, you will see the word "boost" appear at the top of the display and the Drive and Level parameters will now control the Drive and Level for the Guitar Boost.
In this way you can switch back and fourth between the Amp parameters and the Guitar Boost parameters by hitting the assigned button on the Switch-3.