Are you on the £20 goodybag? if so have you used up the first 6GB of data? As the £20 goodybag gives you 6GB of UK data at full 4G speeds and once you used up the 6GB then you will continue to get unlimited data allowance at a reduced speed (256kbps) from 8am to midnight. From midnight to 7.59am you have an unlimited data allowance at full speed.
If you have not used up the first 6GB of data or if your not on the £20 goodybag then i suggest you check if there are any problems with the mobile signal & mast issues in the area please see this link www.bit.ly/ggservicestatus and enter your post code.
If the network signal is fine please try turning your device off, waiting 5 minutes and then turn it back on to force the network signal to refresh and reconnect.
Try change network for normal speed on 4G and 3G. Gg didn't provide usable speed on hours when 99% people using data (7am to 10pm). Or use your data on night time only ( it isn't joke - it is giffgaff - network run by you on night..) 😃