The Moxon is a 2-element yagi with a typical free-space gain of about 6dBi,
or close to 4dB over a dipole. You can get more that from some other 2-element
designs, but they tend to be plagued with poor F/B and/or low feedpoint
If they were claiming 3.3dB over a dipole, that probably is pretty accurate.
The G5RV will have multiple lobes on 10m: the Moxon will be perhap slightly
better in those directions, and much better at filling in the nulls between them.
(If your G5RV is sloping then the Moxon will make even more of an improvement.)
A lot of your signal is eaten up in the tuner with a G5RV.
More likely loss in the coax rather than the tuner. This can be 6dB or more on 10m,
as VK1OD shows here: http://www.vk1od.net/antenna/G5RV/index.htm