Brachot such as a "Hatov Vehameitiv" and "Shehecheyanu" reflect a genuine human emotion. Therefore, one may not recite them unless there is genuine happiness. On the other hand it is possible that if a new situation arose today that was a cause of happiness, one could recite these brachot.
It is probable that in the time of the gemara and rishonim the birth of a daughter was far less significant than the birth of a son. It seems the cultural attitude today has changed, and for the most part people are happy when they have daughters. Based on this cultural difference, it is the opinion of some halachic authorities that if one relates to the birth of a daughter in the same way as the birth of a son, the response to both events should be identical.
Therefore, if you are blessed with the birth of a girl, and it is a joyous occasion to you and your spouse, you may rely on the halachic opinions that state that there should be no difference today in the bracha for the birth of a boy or a girl, and recite "Hatov Vehameitiv". For further reading on this issue see:
Rav Dr. Ephrayim HaLivni, Hadarom 65, 1996, pp. 20-22.