I’m assuming you’re using Serato, which does allow you to map the gain control to something else. Although I don’t know if Intro allows this. Otherwise, the MTP3 is a controller aimed at beginners who rely on Auto gain.
Assuming that you don’t already have a more Professional oriented controller and not having a gain knob is a deal breaker, you can either sell it and/or start saving up for something like the Pioneer DDJ-SR (or SB2 if you still want an intro controller), Reloop Terminal Mix 4/8, or something along those lines.