I have never used Grout Master on travertine, but it should be OK if ti is rinsed well. It certainly cleans good.
My preference is to use a product made for stone that has no chelating agent. Spinergy Stone Perfect is my choice. But, if you have Grout Master and don't do stone often enough to get a product just for stone, then use Grout Master. Rinse well with water.
I just did 1000 sq. Ft. Of travertine. If you have any Harvard retailers new you, they make a product for travertine called 1shot. Works great.
Biggest problem I had with travertine, is it didn't absorb as much sealer as I would have thought. I had to go back and buff it with a white polishing pad to get the excess sealer off.
We did this one yesterday, customer did not want a high shine so we honed it (which really cleans the heck out of it and makes it a natural matte finish. Used a microfiber mop to spread a clear water base sealer, then buffed with a white pad.
. Who ever did it in the past used that 511 crap, I am not a fan of it.