question to crypto experts

Hi,

not directly related to gpg...

AFAIK short messages are weak to build a strong message, but my question 
is related to kbdx files, that are widely used by password managers

the specs are available at : https://keepass.info/help/kb/kdbx.html

basically, this is an encrypted xml file with an header

my question is : knowing that the file will begin with "<xml" does it 
make it weak to decryption attack, and can it be used to get the key?

if yes, how do gpg protect a single file in a way that an attacker 
knowing a part of the file can't decypher it?

thanks and regards

JLM