We have an outdoor receptacle, under an enclosure and with flaps that drop down over the plug area, but not GFCI. We would like to use it for a light sensor timer, with 3-outlet powerblock. Is this safe or should we change out the receptacle?
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.