For this type of testing you might simply use tablet with a software or PC sound card..
All you need to do is to calibrate level and you're good to go..
And as TurboTom already said, impedance on guitar amplifier is a finicky thing..
Passive guitar is a complex impedance, connected with cable that is complex impedance, to an amplifier input that is complex impedance.. They all together define sound, by creating all kinds of resonances and filtering of various frequency bands..
You cannot trivially simulate guitar impedance, well you could do it, but you would need equivalent simulation network for every guitar you would like to test for...
But that is not important. What you want to do is to characterise frequency characteristics of amplifiers (pedals) you like with your generator and then test your design with the same generator..
That would, in theory, allow you to create similar characteristics... In practice it's not that easy..
In this context, pedal input is preamp input you connect guitar to...