Just set up your GI render the way you want it, this is great because you can use lower sampled GI by reducing quality for the GI cache file, or 'mega-sample' it by increasing the render/GI/detail/ray detail multiplier for the GI pass. Once you have written the GI cache, select 'read from file', choose the GI cache in the dialogue and set the dropdown to 'one file'. GI will now be calculated from the saved cache and you don't need to render the prepass each time. Readjust your render settings to where you like again for a final render and the GI will always be the same, whether rendered in crop tiles or not.
* Don't change GI settings midway through an animated sequence's GI cache, the frames can't be blended if they use different sample quality etc. You will get a warning popup but TG will continue to render, just without GI.