We know that both GRE and IPIP encapsulate an IP packet into another IP packet, we know neither GRE nor IPIP provide encryption. But what is the difference between them?

Here it is:

IPIP is really just another IP header on top of the original IP header, whilst GRE apart from traditional IP header data inserts GRE header that may contain checksum, authentication key, control the number of encapsulations, packet sequence, etc. The overhead for IPIP header is 20 bytes versus 24-36 bytes for GRE (the actual overhead depends on which GRE header data is being carried).