This bug is related (but not a duplicate --- as it pertains to Greek rather than Cyrillic) to bug #784549.
The Greek Gamma (capital gamma) character is badly designed. Gamma should not have a serif at the bottom, especially given the design principles of the Ubuntu Mono font. In serif fonts, Gamma should have a very small serif (or a microserif).
As currently designed, the greek Gamma seems alien to anyone familiar with the Greek language/script, is æsethetically incoherent with the general design of the font and is doing your otherwise admirable effort in designing a great looking 'libre' monospace font a disservice. I do realise that the designers of the font may not be familiar with Greek or the Greek script, but as it stands Gamma makes Ubuntu Mono unusable.
Please remove the bottom serif from this glyph.
1. Remove bottom serif from monospace capital Gamma
2. Redraw as two-stroke character, proportioned for monospace Greek usage.
3. Get Yanna on the Canonical Design team to check over.
4. Proposed solution on Canonical Design blog for questions