Bro for the GMT bezel take a look at my bezel removal guide.
6-digit Sub crystal and bezel swap
You should get the 4 blade tool. And get a gap started with a razor blade. Sometimes the bezel gasket (hytrel ring) is so tight that even with the 4 blade tool it won't pop over the retaining ring lip and when you remove the blades the bezel slides back down into place. So what you can do is while the bezel is lifted up with the blades inserted, place a wedge in the space (cardboard, plastic, etc). Then take the watch head out of the tool and work on it with a case knife.
I've never tried to remove a Seiko bezel but rotating bezels almost always use either a gasket or an octagonal wire circlip in a groove. The circlips can be a royal PITA to deal with, depending on the gap between the bezel and the retaining ring. On many lower end Sub reps the wire is visible in that gap. On some it is not. If you can get to the wire end, it can be pulled out one segment at a time. I've had to hammer a small screwdriver into that space to get access to the wire.
Another time I had to just force bend the bezel up over the wire to get it off using the 4 blade tool, some wedges, and then a case knife. Bezels are fairly easy to bend back into shape as long as you they dont get out of round. You place them on a perfectly flat surface and mark a high spot. Then put it on a board with the high spot hanging over the edge and beat the high spot down with a rubber mallet. Repeat til it's perfectly flat again. Most bezels can take some real pounding, especially if the bend is slight and only in a small arc. This method has always worked for me, including on gen 5 digit bezels, which are easily bent.
If the Seiko has a gasket, a different method you can try first before bending the bezel off if it comes to that is melting the gasket away. Different plastics have different melting points, some nylons melt in the lower 400° F range, others around 500. I often use fishline as a gasket replacement and even with it's relatively high melting point I've been able to get it out this way. About 20 - 30 minutes in a 500 - 550° oven will do it. Remove all the O rings and get the crystal gasket out if you want to save it. Melting plastic doesn't usually leave a mess, it just shrinks, cracks and turns brittle.
So try the 4 blade tool on both first, use a wedge if needed then a case knife. If none of that works you can move on to the more aggressive tactics.
