Clean your ProgramData and Users folder
Merge this unallocated space with D drive.
What is in Microsoft folder?
Customize option should be there for GIMP.
I disabled pagefile, reduced hiberfil, deleted unnecessary restore points and merged the unallocated space on the D drive. Gimp did indeed have a custom option that enabled me to save to E drive. I also cleaned up ProgramData and Users folders. There were quite a few files that I deleted after carefully considering whether they were needed or not. I have over 29GB free on the C drive now.
The Microsoft folder had AndroidNDK64 and AndroidNDK folders which I think may not be needed either since it has no use for me. This laptop was passed on to me by someone who had a lot of programming material and programs saved that I don't need at all.
WinDirStat was really very useful while I went about all the cleaning tasks.
I had changed to storage settings to D and E drives so that the C drive can now be used for the OS updates etc. only and not for saving other files and programs.
Thanks for your help!