What is G-CODE?
Many DVD Combo units (as well as most DVD recorders and VHS recorders) allow you to set times to record TV programs while you're out and about. The classic problem with this is that many people find working out dates and times and navigating complex timer menus more than a little baffling. G-Code is intended to be a solution to this problem, as it breaks down the suggested transmission times for programs into a numeric code that can be simply entered from the recorder's remote. You just pick up a TV guide (online, or from a newspaper or magazine), find the program you want to record and enter the number sequence printed next to it. Not every combo drive will support G-Code, so if you're often bamboozled by timer settings, it's a worthwhile feature to look out for.
What can I record?
Whether or not you can record DVDs to VCR depends on the features of your player -- some offer genuine two-way recording while older units may only record in the VCR to DVD direction. That aside, there are copyright and technical limitations to what can be recorded in either direction. Commercial Videotapes produced after the early 1990s tend to carry Macrovision copy protection to deter pirates, and virtually all DVDs carry a similar copy protection system called CSS to stop illegitimate copies being made. VHS Macrovision normally just scrambles the colour and sometimes the audio properties of the signal being sent, meaning that at best you'll get an awful copy of a tape. Many DVD recorders are able to "spot" Macrovision encoding and will simply refuse to record Macrovision-protected material. Likewise, recording from DVD to VHS -- which in itself would involve a considerable loss of visual quality simply due to the lower visual quality of VHS -- will either result in an unwatchable picture or an outright refusal to record.
So what can you record from DVD to VHS and vice versa? Anything that doesn't contain these kinds of copy protection mechanisms and that doesn't contravene existing copyright laws, which basically means any home movies you've shot yourself, whether they're on VHS tape or recordable DVD. This is a big hook for DVD Combo drives, as it means it's very easy to run off multiple copies of, say, a family wedding and distribute copies to relatives in formats they're able to watch.