Here's a versatile "Multi-Trigger" generator with variable pulse, manual trigger and continuous trigger outputs. It also has an LED to monitor the output state of the generator. I have used this very successfully with the GNOME and it should work with the 2720. Also I have found that calculator keys available from Radio Shack and Poly Paks make very good external triggers for the Gnome.
This circuit is basic and can be hardwired on a small piece of perf board. The 555 timers are available as surplus from many of the Mail-Order firms advertising in trade magazines such as Popular Electronics, Radio-Electronics, Audio Amateur, etc. The 556 is a dual 555, and can be used to build up a dual trigger generator.
Using this circuit as a repeating trigger unit, strumming or repeating envelopes can be produced. Or, by hanging a pot on the output, the circuit can be used as a pulse wave control oscillator for VCO trilling and other specialized applications.
If you haven't read much about the 555 type timer circuits, I strongly recommend the articles in February and March 1976 issues of Radio-Electronics, or the spec sheet/applications notes available from the manufacturers of these circuits. There are a number of interesting things that these circuits can do and many of them apply to electronic music.