Is it safe to use an outdoor receptacle that isn't GFCI-protected?
You have asked the question in a way that indicates you are in North America. As such, you should consult the relevant electrical code in your area as to whether ground-fault protection is required for outdoor accessories.

In terms of safety, non-GFCI outdoor accessories are not inherently unsafe, however it may be a legal requirement for new installations, and with good reason. Environmental factors make the risk of physical damage and water ingress greater, increasing the danger of electric shock. A GFCI device may reduce the danger posed, but as has been mentioned, you may already have GFCI protection before the outlet itself.

If not, a like-for-like replacement of the outlet with a suitable GFCI version should be a simple job, but be aware they take up more depth in the mounting box. Investigate the existing outlet before purchasing a new product.

Similarly, you may be able to use a power block with a GFCI-protected plug. These are bulky, like you might see on a hairdryer, and would not be suitable for leaving connected, which may be your intention.

If you do purchase a GFCI outlet, be sure it is also weather resistant and consider ingress protection by way of a waterproof mounting box if the existing outlet is not mounted in such, like this: