I do own both Gatco and Lansky.
Lansky has the advantage, that "both" angles (i.e. upside and downside) will be identical but the disadvantage, that the given angle values are "false" (they are about 4° less than given).
Gatco has the advantage, that the given angle values are "correct", but the upside and downside angles differ a little bit, due to the construction of the jig, depending on blade thickness.
NOW: both systems require that you know how to clamp your knife! Both have a tiny notch
(abt 1/4") for small knives. If you use it, the knives will be in the same position for every sharpening session. That's good.
But: for large knives holding force will be insufficient. I use the following rule: The blade ist to protrude (exactly) 1/2" while sharpening. Furthermore I keep book on every sharpening session. It helps when you sharpen the next time (might be months away...).
For VERY large knives I use the edgepro pro sharpener, expensive, but the best.
The chosen angle is always the next below a sharpmaker (i.e. 30° or 40°) so I can use the sharpmaker to get ultimate sharpness AND use it for maintaining this sharpness until next honing session is required. (after MANY upkeeps!)
Never forget to use a colormarker on the edge to identify the angle with the first (easy!) stroke. It prevents hours of regrinding in case of an error!!!
For more details e-m@il if you wish.
Happy sharpening.
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D.T. UTZINGER