the GUMSHOE is now available under two open licenses; the Open Gaming License and the Creative Commons 3.0 Attribution Unported License.
That means you are free to choose which license to use. The OGL license is a bit more complex, but the CC-BY-3.0 Unported has a very simple "human-readable" synopsis:
You are free to:
Share — copy and redistribute the material in any medium or format
Adapt — remix, transform, and build upon the material
for any purpose, even commercially.
The licensor cannot revoke these freedoms as long as you follow the license terms.
Under the following terms:
Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.
That means you can use it commercially, you just need to add an attribution note to your work which says that it is based on the Gumshoe ruleset by PengalPress Ltd. and that that ruleset is licensed under CC-BY 3.0.
Your own contribution to the combined work are not affected by the CC-BY license.