It is about 5" by5" in a dark brown frame.It has very old backing paper taped down.It is definitely oilpainted but does not have glass over.It has a taped message saying Thomas Gainsborough the blue boy and a date of 1772. . I bought from a charity shop today.It has no signature on it

I doubt that the message and the painting were done in 1772. The original was supposedly done in 1770, but there is no proof of the exact date.
You can take off the brown paper, which might be a lot more recent than you think, despite looking old. Tell us what you see for the back of the painting, any writing, the nails or wedges, anything.
Are you positive it's oil? (BTW glass would never be put over oil)

I'm thinking reproduction too

Reproduction isn't the issue (the original is huge, and hangs in the Huntington).
I think the question is when was the copy made, and is it an oil.
If it's old enough and good enough, it might be worth a little bit.
I have a few copies of famous paintings hanging in my living room - painted by me.

I was thinking reproduction as in mass produced print.
All the miniatures I have seen are oval and worth less than $10.00
Posting a pic would help a little

True