The Sonuus G2M is the least expensive and also the simplest way to get MIDI from your guitar - it takes the signal from the guitar's jack socket and converts it directly to MIDI.
The downside is the G2M is monophonic, so you can only play single notes, unlike a hex pickup system that detects all six strings.
The G2M is great for recording anything monophonic, and you can just plug into a synth and play its sounds or play hybrid synth/ guitar via the G2M's thru jack.
If you want pitch-to-MIDI conversion and aren't bothered about chords or don't want to stick a hex pickup on your guitar, the G2M is perfect.