G50 is the average score expected from the team batting first in an uninterrupted 50 overs-per-innings match.
This will vary with the level of competition and over time. The annual ICC Playing Handbook gives the values of G50 to be used each year when the D/L Standard Edition is applied.
245 is used in matches involving full member nations and in FCC (noting DL is only used in FCC if DLS is not available)
200 is used as a default in other lower level matches.