The easiest way to do this if your form isn't already linked is to start again and use the form wizard.
If you use the form wizard, it will ask you for the source data, which can be a table or query. Go through the wizard steps and if the end result isn't exactly what you want, go into design view and change the fields and locations.
If you don't want to use the wizard, and want to design the form anew, then you would select new form and make sure you choose the correct source data, i.e., table or query. Once you finish that process you will be in the design mode of the form. If you pass the cursor over the icons in the menu bar, you will find one that says Field List. Select that and all the fields that you want appear in the Field List Box. Drag the fields you want to the form. They automatically default to Text Boxes, you can change this by right-clicking and there should be an option to convert to another Form object, i.e., combo box. If you are using 97, I don't know. What you can do is look at the menu bar listings and see if there is a convert method.
If you still can't convert to a combo box for the GATE field, delete the TextBox for the GATE field and select the combo box from the toolbox toolbar as discussed previously. Go through the wizard and it may ask what the control source is, if it does, then it is easy, you just select the GATE field from the table you setup and your done.
I'm rambling, but you get the picture.
Lastly, if you wanted to link the form that you have as it is, you would have to select the appropriate CONTROL SOURCE for each form object. You would right-click on every object and goto control source, press the ... button that appears when you place your cursor in the CONTROL SOURCE property and then another dialogue box will appear Expression Builder. All you want to do is select the Table from the lists below the blank screen for expressions. Once you doubleclick the appropriate field, the blank sceen will have something like =GATE
let me know if you need anything else...