If Rav Gidel would only have told us the second saying; that if he said he will give the erech of a hand he must give the value of the hand according to Rabbi Meir; we would not have known the first saying, that if he said he will give the erech of the vessel he must give the value of the vessel; because we might have thought that even according to Rabonan he has to give the value of the vessel (as the Gemara states above 5a, because everyone knows that a vessel has no erech, so he must have meant the value). Therefore we need both sayings of Rav Gidel, so that we should know that Rav Gidel only said his sayings according to R. Meir.

KOL TUV

Dovid Bloom

Further answer:

Mesivta answers this question in the name of sefer Mutzal Me'eish.

Mutzal Me'eish writes that if Rav Gidel would have only stated the din that if one says the erech of a hand is incumbent on me, I might have thought that the reason according to Rabbi Meir that he has to pay the value of the hand, is similar to what the Gemara states above 5b; that we might have thought that the reason one has to pay its value if one says that the erech of a baby less than a month old is incumbent on me, is a gezera that if one does not pay for a baby less than a month old, one may come not to pay even for people over a month old. Similarly, if one would not pay when one says the erech of a hand is upon me, one may come not to pay even if one says the erech of the whole person is upon me. If this would be the reason; because of a gezera; then if one says the erech of a vessel is incumbent upon me, there would be no need to make such a gezera beceuse no-one would get confused between a vessel and a person, and it would be obvious that if one does not pay if one says the erech of a kli is upon me, one would not come as a result to avoid paying if one says the erech of a person is upon me. Therefore if Rav Gidel would only have said the second saying, we would not have automatically known the first saying from the second.

Dovid Bloom