how to lower gb in photo

If you want to have the same pictures take up less space on your Mac, then the best plan is to use iCloud to help you out. Photos will store you pictures in iCloud and keep smaller versions on you Mac that look the same but don't take up nearly as much space. When you want to edit or print a particular picture, Photos will retrieve the full scale image from iCloud, usually without you seeing anything change, and let you work with that.

To use iCloud, you must, in preferences, tell Photos to use iCloud, and also check the box that says "Optimize Mac Storage." You will probably have to give Apple some money to have enough room to store your pictures in iCloud.

The details are here:

Set up and use iCloud Photos - Apple Support

Another idea would be to get an external drive (it can't be the one on which you have Time Machine) and transfer your Library to that drive. The external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Then you just drag the Library to the new drive and tell Photos, in preferences, where the new one is. See this:

Move your Photos library to save space on your Mac - Apple Support