Hello!
The "dedicated" GTR input on the 414mkII is nothing more than an extra input with an extra "pad" resistor,... to handle the relatively hotter signal you'd typically get off a e'guitar pickup, vs. the "standard" line level signal from other typical "line" devices.
It's also a handy & cool feature, that makes the 414mkII a cut above the tech of the 414 (std).
You should not need a separate DI box for guitar, when using the 414mkII or any other similar Tascam Portastudio. Use the built-in inputs and GTR input, and you'll be fine. Plug the guitar into the Porta' & go to it!!
PS: any of the other 4-(main)-Inputs will accommodate an e'guitar signal just fine, too. Think of the dedicated GTR input on the 414mkII as "icing on the cake".
Thanx, & good luck!
/DA