You can tile over gyproc ok. You didn't say what the rest of the wall was made from as this can have a bearing on tile fixing. If you are out of level by an inch as you say, you may need to pack out underneath where the gyproc is going to be fixed to bring the surface level as gyproc is typically only 12mm thick.
If you are to be tiling into a wet area you may want to consider a cement based backer board instead as these are much more water resistant.
You didn't give much information about the type and size of tiles you wish to fix in your question and this can have an impact on your job requirements for adhesive etc.
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Grumpy