Where can I find the actual radiation pattern and signal strength of transmitting antennas used by GNSS navigation satellites/constellations (GPS, GLONASS, IRNSS, etc)?
Exact radiation patterns and gain (you're probably not asking about signal strength since that depends on distance which isn't constant) will vary across all the different GNSS contellations, even individual generations / blocks of same systems, but for a fairly detailed analysis of GPS blocks, you can refer to e.g. GPS Space Service Volume: Ensuring Consistent Utility Across GPS Design Builds for Space Users.
For example, side lobe gain patterns could look like this (for GPS Block IIF and Block IIR-M):
And hemispherical gain patterns could look like this (For GPS Block IIR):
You can find more relevant information e.g. by searching the NASA Technical Report Server (NTRS).