Difference between in-band and out-band 3rd order intercept points?
The datasheet is pretty good on this - they give you the exact frequencies used in the IP3 tests.

The in-band IP3 refers to two tones both in the band of interest, which would be for example weak TV harmonics, or an intentional jammer, interfering with, and distorting the signal.

The out of band IP3 refers to two tones well out of band, mixing into the band. They've chosen a pair of cellphone signals which could intermodulate to 1575 MHz and would interfere with the receiver. So while both tones are far away, the mixing product is in-band.

There must be some modest band-pass filtering in front of the active element to achieve this. They need to specify it as two different IP3 figures, as simply stating one inputIP3 and the gain would not deliver the same benefit in the RF chain linearity analysis.
If you're analysing linearity against in-band jammers, use the in-band figure, and if youre analysing it against cellphone signals, use the out-of-band figure.

You can filter out the out-of band frequencies, but you need to do it before the amplifier, and this will affect the noise figure. Up to you as the designer!