lets start by saying that gradients are normal as the sky has different levels of illumination, light pollution both Lunar and man made etc. It is not a fault or bad data and is expected to be dealt with as a normal part of you image processing routine
It will manifest itself as unevenness across some of the frames. This is perfectly normal as we allow for small amounts of gradients caused by moon despite our moon avoidance algorithms as it is impractical to to only image when the moon is down. Our moon avoidance algorithms will reduce the amount of influence from these types of gradients but unfortunately will not completely avoid them.
There are many ways to correct for gradients like these, the most powerful in our opinion is the DBE tool in Pixinsight. It will completely remove it from your data.
We hope this helps
Please ignore my dylexia wherever possible, just be thankful I can control my Tourettes ;)
Things to do, so little time!