In my practical experience, yes it does work that way, the Galileo does "take a free ride" as far as GPS L1 antennas are concerned. (The receiver is a different story of course, because the modulation and coding schemes are different.)
A good GPS antenna contains a ceramic filter for "out of band rejection" that's typically about 60 MHz wide and pretty steep. A single stage ceramic filter has about -50 dB suppression outside its passband. I've seen antennas containing a cascade of two filters to achieve about -70 dB "out of band suppression".
Navipedia has some nice GNSS frequency plans / diagrams for GPS, Galileo, GLONASS and BeiDou.
A GPS antenna should work for Galileo in terms of signal levels too (amplitude and gain). To be more specific, I believe the signal levels at antenna "driven element feed point" are in the ballpark of -130 to -160 dBmW, which means that a couple dozen extra satellites (Galileo) won't overload the preamp and intermodulation is practically not a factor either.
Note that this compatibility probably works for "direct amplified" L1 antennas only. In contrast, antennas containing a down-converter (mixer) are likely to contain a narrower filter in the IF stage. But, such antenna downlink systems are proprietary and will be vendor-locked to a particular receiver. Thus, it's ultimately a matter of the receiver anyway, if Galileo is supported or not... I guess I know one such proprietary system where the receiver (demodulator) really decodes the GPS L1 C/A code only, and therefore probably only needs about +/- 2 MHz from the center frequency, out of the rather complex GPS L1 spectrum that encompasses more like +/- 20 MHz. Those receivers are niche timing stuff, really - with long coax antenna downlinks. Not sure how the modern, highly integrated positioning receivers are constructed, if they work with analog IF downconversion at all, or if the front end uses direct carrier sampling and discrete time down-conversion, hinging on artful abuse of Nyquistian aliasing... (wifi style)
==== EDIT ====
oops, you've included the signal plans already, apologies for stating the obvious then :-)
