Silly mistake - forgot pattern lock - out of options?

ADB you will need to get working on your computer. With most Linux distros this is fairly easy (sudo apt-get install android-tools-adb android-tools-fastboot), other OS may need a bit more effort (see here for a tutorial, or for example for Windows).

With ADB & fastboot in place, you can flash the above mentioned TWRP (fastboot flash recovery TWRP-filename.img), which might be already enough to give you access to change the above mentioned file. Yet, Rather than deleting, I would recommend to rename the file, so you could put it back in place if necessary. Thus, instead of rm /data/system/gesture.key it would be better to use mv /data/system/gesture.key /data/system/gesture.key.bak (which only renames the file to /data/system/gesture.key.bak).

So basically the main step would be to get ADB/fastboot running on your computer. This I would recommend any user of an Android device, because it can help so much with troubleshooting, but also for backing up and copying data, as well as for installing software (custom ROMs or OS updates).