Since it's been running OK w/o a GFCI, you can assume that there is not a direct short, or the breaker would have tripped. So you're looking a something like a 5ma+ leakage or a bad GFCI.
The new motor could have a ground fault, though it's less common on new motors. Older motors, sure, because the insulation tends to degrade over years, and accumulations of oils, dust, ants, oxidation, etc. can allow more and more leakage over time.
When you have an electrical fault, you kind of have to assume that EVERYTHING can be suspect, and rule out each in turn. Anywhere there is a connection along the wiring path can be faulted, and the wire itself could be cracked/skinned/broken somewhere along the way. (the wiring can be ruled out by bypassing with a temporary run above ground between the GFCI and the motor - just to test).
If you can rule out everything up to the motor, then the motor is the probable suspect.
As far as the bonding grid is concerned, it is an interconnection grid designed to keep all wettable components within a given distance from the pool at a single voltage potential to prevent any potential for electric shock from touching two different things, like the water and the ladder for instance. You can do a quick search of the forum using the google search at the very bottom left of each page for the term "equipotential" and probably get a good selection of discussions about it.
Also, where are you located? The requirements are different in different areas, like US to Canada.