I would strongly advice against thinners. There are paint strippers for GRP, but by definition the chemicals are less aggressive and so it's harder work. I would try a marine supplier.
Are you trying to remove the paint for a clear finish or just to respray?
If for painting a light sand should be sufficient. Stop if you see carbon, as you do not want to erode the matrix and it is important to keep moisture away from the fibres, as otherwise it breaks down be bond between fibre and matrix.
