Capabilities.java

public class Capabilities {
  private int alloy;
  private Range gauge;
  private Range width;
}

Range.java

public class Range {
  private int value;
  private String range;
  private boolean isRange;

  public Range(int value) {
    this.value = value;
    this.isRange = false;
  }

  public Range(String value) {
    this.range = range;
    this.isRange = true;
  }

  public boolean isRange() {
    return this.isRange;
  }

  public int getValue() {
    return this.value;
  }

  public String getRange() {
    return this.range;
  }
}

RangeParse.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class RangeParse {
  public static void main(String [] args) {
    Gson gson = new GsonBuilder().create();
    Capabilities capabilities = gson.fromJson("{\"alloy\":1100,\"gauge\":\"1.0-2.0\",\"width\":1.0}", Capabilities.class);
    gson.toJson(capabilities, System.out);
  }
}

RangeTypeAdapter.java

import java.lang.reflect.Type;

import com.google.gson.JsonSerializer;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonParseException;

public class RangeTypeAdapter implements JsonDeserializer<Range>, JsonSerializer<Range> {
  public Range deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext ctx) throws JsonParseException {
    String range;
    Integer value;

    try {
      value = json.getAsInt();
      return new Range(value);
    }
    catch (NumberFormatException e) {
      return new Range(json.getAsString());
    }
  }

  public JsonElement serialize(Range range, Type typeOfSrc, JsonSerializationContext ctx) {
    if (range.isRange()) {
      return new JsonPrimitive(range.getRange());
    }

    return new JsonPrimitive(range.getValue());
  }
}