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I've just bought one of these https://www.giant-bicycles.com/us/anthem-x-29er-2-2012.
Will a shimano push fit BB fit a hole where a GXP one was ? I intend to use an XT crankset that currently uses an external bottom bracket . Will that be compatible will a push fit XT BB?
For fitting do I need a BB30 tool?
Will the sram  chainset fit on the bike with the external shimano BB?
In these days of everything being different sizes I thought I'd better check.
I have just fitted a SRAM DUB PF BB to a mates 2014 Anthem 29er. It had a GXP BB previously.
I can't see why a 24mm Shimano BB shouldn't fit but not too sure where 30mm is involved.
When I fitted the DUB BB, the centre tube would not fit, so that got left out.
If you were going to PF30 you might have to forgo the centre tube.. I think that BB30 is/was a SRAM standard.
A Shimano crankset will not work in a GPX BB.
Good luck.
Paul
Yes it'll fit, a threaded rod with big washers or some pieces of hardwood can be used or a vice if you don't have a bb/headset press, just protect surfaces and make sure it goes in straight. Gxp cranks need a spacer to work with shimano bb, you can get them off ebay.