I'm new to Lightroom so please bear with me.
I've imported a number of images which I've then tweaked the exposure and a couple of other settings. I then would like to keep these changes (each image is different) and then apply a Greater than Gatsby preset to them on top and then export, but I can't figure out how to do it, and applying the GTG preset means it forgets the tweaks I already made to the images.
That's the way presets work. They are not in addition to your existing changes, they replace existing changes.
You might want to try applying the preset first, and then additional exposure and other changes.
Ah,thank you, so if I want to apply different presets to images within the same library though? Is there a way around this?
If you want the presets to be cumulative (for example, one uses exposure of +0.25 and the next preset uses exposure of +0.5, with the desired result of +0.75 exposure) there is no way to achieve this using presets.
If you have two different presets that affect different develop sliders (one adjusts exposure, and the second one adjusts saturation), then this does work, both effects will appear on your photo.