To create an Marshall Gear tone, you must have the Lead Guitar career path chosen. To do so, press Back at the main menu, and move the slider to the Lead Guitar career path and press A.
Now, chose Tone Designer from the main menu. From here, all you have to do is change one thing on the tone and save in order to get the achievement. This can be done by selecting Edit Tone, selecting any option (Rack for example), then edit one slider and select Finish. Now press B to back out of this menu, select the Finish option, and the game will ask you to save the tone. Once named and saved, the achievement will pop.
Rinse and repeat under the Bass Guitar and Rhythm Guitar career paths for the other tone achievements.