Hi Kolif,
Gilts' and 'Gilt strips' are not subject to Capital Gains Tax.
While waiting for the the gilt to mature, the investor is paid interest at a fixed rate, known as the 'coupon'.
This is reported as interest on a self assessment tax return (SA101).
The difference between the buying price and the selling price is also interest and is declared on SA101, boxed 1, 2 and 3 of page Ai1.
Have a look at HS343 to see if the accrued income scheme is appropriate.
Accrued Income Scheme (Self Assessment helpsheet HS343)
Thank you.