I don’t believe this is a Freetrade issue; the UK government would be crazy if they let anyone do this. “Oh, look that stock is doing really well! Let’s just remove my tax obligations entirely.”
Apologies for the sarcasm, but I think almost everyone who could bypass capital gains tax by easily transferring their profitable shares to a S&S ISA from a GIA would do so. As mentioned, you can sell all your GIA instruments and then buy them again in your ISA. You can ask Freetrade to transfer the money for you, rather than withdraw it yourself and deposit it again. This is a manual process and would probably ordinarily take as long as it would for you to withdraw, receive the money and then deposit again. Right now though, customer service is busier than usual so I’d suggest you do it yourself.