Thank you for contacting the GiffGaff Community. Hopefully the following information will help to assist you further.
You could first try a Manual Network Roam within your phones Mobile Network Settings to connect to a different Network. https://community.giffgaff.com/t5/Using-giffgaff/How-to-perform-a-Manual-Roam/ta-p/4871794
You might not be able to use the 4G network while you're abroad due to differences in the frequencies used, but don't worry - you can still use your 4G device on 2G and 3G.
Similarly, if friends from overseas visit the UK with a 4G device, they'll be able to connect to the 2G and 3G networks. But they won't be able to roam on to 4G because of differences in frequencies.
On most devices, switching between 4G and 2G/3G should happen automatically but for some networks in certain countries you may have to manually turn off 4G and use 3G instead. Once you've done that, you can manually select a network from a list on your phone. Manual selection of your network applies in the following countries.How to manually turn off 4G
Follow these set up instructions so you can connect to a 3G network while you're abroad.Samsung
Tap apps > settings > more settings > network > network mode > select 3GHuawei
Tap apps > settings > more > network > network mode > select 3G only
Tap settings > general > cellular > enable LTE > switch to the OFF position
Tap settings > mobile network > network mode > 3G only
REMEMBER TO SWITCH IT BACK TO 4G WHEN YOU RETURN TO THE UK!