Drag the red marker on the left map to where you want a National Grid reference*. (The blue marker on the right map cannot be dragged). The grid refs will appear on the right, and the corresponding boxes are drawn on both maps.
Please do not record wildlife using 2km, 5km or 10km grid refs as they are too vague. These are for information only.

On the left map press "Satellite" for aerial photos only, or "Map" for Google Maps only.
The Vice-counties, VC30 and Bedfordshire boundaries only show on the left map (at present...)

The right map shows Ordnance Survey (OS) maps of different scales according to the zoom amount. (OS limits the number of map tiles accessed to 100,000 per day, so the right map will stop working if this is reached).

The right map tracks the left map whenever it is zoomed, panned or the marker moves, but the right map can be can be zoomed and panned on its own. The zoom level of the right map can be locked to its current level so that it doesn't change when the left map is zoomed. Just tick the box under the OS map. (The OS zoom can still be changed manually of course).
If you've found the place you wanted using the right map, you can move the left map to it using the "Match Left Map to Right Map" button.
The "Get The Marker" button moves the marker to the centre of the left map, so match the maps first.

Bookmarking: Due to differences between browsers as to how they support bookmarks I have abandoned the previous method. The following is now the best way:
Add "?gr=XXXXXX" to the end the GAGR web address, replacing XXXXXX with your desired gridref, e.g. https://www.bnhs.co.uk/2019/technology/grabagridref/gagrol.php#map?gr=TL123456 will go to TL123456, then use your browser's normal bookmarking method to remember it for future use. (Remember to actually "go to" each newly created web-address containing a new gridref otherwise your browser will just bookmark the previous address again).

The "Go to ..." box can be used to go to a Grid reference (e.g. TL123456), or a postcode e.g. (MK41 9DJ), or a place name. (Currently it goes to the first matched place name, but a choice may eventually be offered - bear with me).

The "Toggle Vice-County" selector turns a Vice-County boundary on and off each time that county is selected. NOTE: It takes a while to download to your PC the large files defining some VCs, so you may see a significant delay before a boundary appears - please be patient. Multiple VCs can be displayed. If you want to clear all the VCs, select the second option "Clear Vice-Counties". Clicking on a boundary will tell you which VC it is.

GAGR fits to the browser's window size when started. If you subsequently change the browser window size you can re-fit GAGR to it using the "Fit to Window" button (top-left).

Accessible Wildlife Site (AWS) and Roadside Nature Reserve (RNR) boundaries courtesy of the BRMC which maintains them. Note that these boundaries are intentionally only approximate and should NOT be treated as definitive site boundaries.

* as defined on Page 35 of A Guide to Coordinate Systems in Great Britain
(Although the National Grid covers a portion of Ireland Irish grid references are usually given using a different system. Irish grid refs can be obtained here)